2016-09-09 18:17:25 +02:00
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{-# LANGUAGE DefaultSignatures #-}
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{-# LANGUAGE DeriveFunctor #-}
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{-# LANGUAGE FlexibleInstances #-}
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{-# LANGUAGE FlexibleContexts #-}
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{-# LANGUAGE OverloadedStrings #-}
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{-# LANGUAGE RecordWildCards #-}
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{-# LANGUAGE ScopedTypeVariables #-}
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{-# LANGUAGE TypeOperators #-}
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-- | Dhall is a programming language specialized for configuration files.
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--
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-- The simplest possible way to use Dhall is to ignore the programming language
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-- features and use it as a strongly typed configuration format. For example,
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-- suppose that you have the following configuration file:
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--
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-- > $ cat > config
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-- > { foo = 1
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2016-09-15 18:34:55 +02:00
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-- > , bar = [3.0, 4.0, 5.0] : List Double
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2016-09-09 18:17:25 +02:00
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-- > }
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2016-09-10 04:02:13 +02:00
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-- > <Ctrl-D>
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2016-09-09 18:17:25 +02:00
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--
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2016-09-10 04:02:13 +02:00
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-- You can read the above configuration file into Haskell using the following
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-- code:
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2016-09-09 18:17:25 +02:00
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--
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-- > {-# LANGUAGE DeriveGeneric #-}
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-- > {-# LANGUAGE OverloadedStrings #-}
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-- >
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-- > import Dhall
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-- >
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-- > data Example = Example { foo :: Integer , bar :: Vector Double }
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-- > deriving (Generic, Show)
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-- >
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-- > instance Interpret Example
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-- >
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-- > main :: IO ()
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-- > main = do
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-- > x <- input auto "./config"
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-- > print (x :: Example)
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--
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-- The above program prints:
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--
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-- > $ ./example
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-- > Example {foo = 1, bar = [3.0,4.0,5.0]}
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--
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2016-09-10 04:02:13 +02:00
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-- In the above code, the `Example` Haskell type represents the schema for our
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-- configuration file. Suppose that we modify our configuration file to no
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-- longer match the schema, like this:
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2016-09-09 18:17:25 +02:00
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--
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2016-09-11 19:55:34 +02:00
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-- > $ echo "1" > config
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2016-09-09 18:17:25 +02:00
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--
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-- This then throws an exception when we try to load the configuration file:
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--
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2016-09-11 19:55:34 +02:00
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-- > $ ./example
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-- > example:
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2016-09-17 05:16:46 +02:00
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-- > Expression: 1 : { bar : List Double, foo : Integer }
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2016-09-09 18:17:25 +02:00
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-- >
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-- > Error: Expression's inferred type does not match annotated type
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-- >
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2016-09-11 19:55:34 +02:00
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-- > Explanation: You can annotate the type or kind of an expression like this:
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-- >
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-- > x : t -- `x` is the expression and `t` is the annotated type or kind of `x`
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-- >
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-- > Annotations are introduced in one of two ways:
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-- >
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-- > * You can manually annotate expressions to declare the type or kind you expect
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-- > * The interpreter also implicitly inserts a top-level type annotation
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-- >
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-- > Annotations are optional because the compiler can infer the type of all
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-- > expressions. However, if you or the interpreter inserts an annotation and the
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-- > inferred type or kind does not match the annotation then type-checking fails.
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-- >
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-- > You or the interpreter annotated this expression:
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-- > ↳ 1
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-- > ... with this type or kind:
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-- > ↳ { bar : List Double, foo : Integer }
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2016-09-11 19:55:34 +02:00
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-- > ... but the inferred type of the expression is actually this type or kind:
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-- > ↳ Integer
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2016-09-09 18:17:25 +02:00
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--
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-- The Dhall programming language is a statically typed language and the
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-- above error message is the output of the language's type-checker. Every
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-- expression we read into Haskell is type-checked against the expected schema.
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--
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-- The above error message says that the type-checker expected a record with
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-- two fields: a field named @bar@ that is a `Vector` of `Double`s, and a
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2016-09-11 19:55:34 +02:00
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-- field named @foo@ that is an `Integer`. However, the type-checker found an
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-- expression whose inferred type was an `Integer`. Since an `Integer` is not
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-- the same thing as a record the type-checking step fails and Dhall does not
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-- bother to marshal the configuration into Haskell.
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2016-09-09 18:17:25 +02:00
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--
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-- Dhall is also a heavily restricted programming language. For example, we can
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-- define a configuration file that is an anonymous function:
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--
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-- > $ cat > makeBools
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-- > \(n : Bool) ->
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2016-09-15 18:34:55 +02:00
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-- > [ n && True, n && False, n || True, n || False ] : List Bool
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2016-09-09 18:17:25 +02:00
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-- > <Ctrl-D>
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--
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-- You can read this as a function of one argument named @n@ of type `Bool`
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-- that returns a `Vector` of `Bool`s. Each element of the `Vector` depends
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-- on the input argument.
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--
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-- This library comes with a command-line compiler named @dhall@ that you can
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-- use to type-check configuration files and convert them to a normal form. For
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-- example, we can ask the compiler what the type of our @makeBools@ file
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-- is:
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--
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-- > $ dhall typecheck < makeBools
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2016-09-17 05:23:12 +02:00
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-- > ∀(n : Bool) → List Bool
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2016-09-09 18:17:25 +02:00
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--
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-- This says that @makeBools@ is a function of one argument named @n@ of type
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-- `Bool` that returns a `Vector` of `Bool`s.
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--
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-- We can apply our file to a `Bool` argument as if it were an ordinary
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-- function, like this:
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--
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-- > {-# LANGUAGE OverloadedStrings #-}
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-- >
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-- > import Dhall
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-- >
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-- > main :: IO ()
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-- > main = do
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-- > x <- input auto "./makeBools True"
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-- > print (x :: Vector Bool)
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--
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-- This produces the following output:
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--
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-- > $ ./example
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-- > [True,False,True,True]
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--
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-- Notice how we can decode into some types \"out-of-the-box\" without declaring
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-- a Haskell record to store the output. In the above example we marshalled
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2016-09-10 04:02:13 +02:00
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-- the result directly into a `Vector` of `Bool`s. The instances for the
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-- `Interpret` class list all types that are automatically supported.
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2016-09-09 18:17:25 +02:00
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--
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-- We can also test functions directly on the command line using the @dhall@
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-- compiler. For example:
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--
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-- > $ dhall
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-- > ./makeBools False
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-- > <Ctrl-D>
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2016-09-15 18:03:12 +02:00
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-- > List Bool
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2016-09-09 18:17:25 +02:00
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-- >
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2016-09-17 05:16:46 +02:00
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-- > [False, False, True, False] : List Bool
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2016-09-09 18:17:25 +02:00
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--
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-- The @dhall@ compiler with no arguments produces two output lines:
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--
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-- * The first output line is the type of the result
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-- * The second output line is the normal form of the expression that we input
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--
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-- In the above example the type of the result is a `Vector` of `Bool`s and the
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-- normal form of the expression just evaluates all functions.
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--
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2016-09-10 04:02:13 +02:00
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-- You can also use the Dhall compiler to evaluate expressions which have no
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-- file references. For example:
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2016-09-09 18:17:25 +02:00
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--
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-- > $ dhall
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2016-09-15 18:34:55 +02:00
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-- > "Hello, " <> "world!"
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2016-09-09 18:17:25 +02:00
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-- > <Ctrl-D>
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-- > Text
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-- >
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-- > "Hello, world!"
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--
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-- > $ dhall
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-- > +10 * +10
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-- > Natural
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-- >
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-- > +100
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--
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2016-09-10 04:02:13 +02:00
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-- Dhall is a very restricted programming language that only supports simple
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-- operations. For example, Dhall only support addition and subtraction on
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-- `Natural` numbers (i.e. non-negative numbers), which are not the same type of
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-- number as `Integer`s (which can be negative). A `Natural` number is a number
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-- prefixed with the @+@ symbol. If you try to add or multiply two `Integer`s
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-- (without the @+@ prefix) you will get a type error:
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2016-09-09 18:17:25 +02:00
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--
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-- > $ dhall
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-- > 2 + 2
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-- > <Ctrl-D>
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-- > dhall:
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2016-09-09 18:51:31 +02:00
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-- > Expression: 2 + 2
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2016-09-09 18:17:25 +02:00
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-- >
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2016-09-11 19:55:34 +02:00
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-- > Error: Cannot use `(+)` on a value that's not a `Natural`
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-- >
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-- > Explanation: The `(+)` operator expects two arguments of type `Natural`
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-- >
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-- > You provided this argument:
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-- >
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-- > 2 + ...
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-- >
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-- > ... whose type is not `Natural`. The type is actually:
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-- > ↳ Integer
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2016-09-09 18:17:25 +02:00
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-- >
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2016-09-11 19:55:34 +02:00
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-- > An `Integer` is not the same thing as a `Natural` number. They are distinct
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-- > types: `Integer`s can be negative, but `Natural` numbers must be non-negative
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-- >
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-- > You can prefix an `Integer` literal with a `+` to create a `Natural` literal
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-- >
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-- > Example:
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-- >
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-- > +2 + ...
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2016-09-09 18:51:31 +02:00
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--
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-- The Dhall language doesn't just type-check the final schema; the language
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-- also ensures that every expression is internally consistent. For example,
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-- suppose that we call @./makeBools@ on a non-`Bool` argument:
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--
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--
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-- > $ dhall
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-- > ./makeBools "ABC"
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-- > dhall:
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2016-09-17 05:16:46 +02:00
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-- > Expression: (λ(n : Bool) → [n && True, n && False, n || True, n || False] : List Bool) "ABC"
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2016-09-09 18:51:31 +02:00
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-- >
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2016-09-11 19:55:34 +02:00
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-- > Error: Function applied to the wrong type or kind of argument
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-- >
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-- > Explanation: Every function declares what type or kind of argument to accept
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-- >
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2016-09-17 05:23:12 +02:00
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-- > λ(x : Bool) → x -- Anonymous function which only accepts `Bool` arguments
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2016-09-11 19:55:34 +02:00
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-- >
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2016-09-15 18:34:55 +02:00
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-- > let f (x : Bool) = x -- Named function which only accepts `Bool` arguments
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2016-09-11 19:55:34 +02:00
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-- > in f True
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-- >
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2016-09-17 05:23:12 +02:00
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-- > λ(a : Type) → a -- Anonymous function which only accepts `Type` arguments
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2016-09-11 19:55:34 +02:00
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-- >
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-- > You *cannot* apply a function to the wrong type or kind of argument:
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-- >
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2016-09-17 05:23:12 +02:00
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-- > (λ(x : Bool) → x) "A" -- "A" is `Text`, but the function expects a `Bool`
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2016-09-09 18:51:31 +02:00
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-- >
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2016-09-11 19:55:34 +02:00
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-- > You tried to invoke a function which expects an argument of type or kind:
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-- > ↳ Bool
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-- > ... on an argument of type or kind:
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-- > ↳ Text
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2016-09-09 18:51:31 +02:00
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--
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-- We get a type error saying that our function expects a `Bool` argument, but
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-- we supplied an argument of type `Text` instead.
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--
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2016-09-10 04:02:13 +02:00
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-- Our `input` function also doesn't need to reference any files at all:
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2016-09-09 18:51:31 +02:00
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--
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-- >>> input auto "True && False" :: IO Bool
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-- False
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--
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-- Reading from an external configuration file is just a special case of Dhall's
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-- support for embedding files as expressions. There's no limit to how many
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2016-09-10 04:02:13 +02:00
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-- files-as-expressions that you can nest this way. For example, we can define
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2016-09-09 18:51:31 +02:00
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-- one file that is a Dhall expression that in turn depends on another file
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-- which is also a Dhall expression:
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--
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-- > $ echo './bool1 && ./bool2' > both
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-- > $ echo 'True' > bool1
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-- > $ echo 'False' > bool2
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-- > $ dhall
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2016-09-15 18:34:55 +02:00
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-- > [ ./bool1 , ./bool2 , ./both ] : List Bool
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2016-09-09 18:51:31 +02:00
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-- > <Ctrl-D>
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2016-09-15 18:03:12 +02:00
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-- > List Bool
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2016-09-09 18:51:31 +02:00
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-- >
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2016-09-15 18:34:55 +02:00
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-- > [ True, False, False ] : List Bool
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2016-09-09 18:51:31 +02:00
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--
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-- The only restriction is that the Dhall language will forbid cycles in these
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-- file references:
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--
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-- > $ echo './bar' > foo
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-- > $ echo './foo' > bar
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-- > $ dhall < ./foo
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-- > dhall:
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-- > ⤷ ./bar
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-- > ⤷ ./foo
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-- > Cyclic import: ./bar
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--
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-- Dhall is a total programming language, which means that Dhall is not
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2016-09-10 04:02:13 +02:00
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-- Turing-complete and evaluation of every Dhall program is guaranteed to
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-- eventually halt. There is no upper bound on how long the program might take
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2016-09-09 18:51:31 +02:00
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-- to evaluate, but the program is guaranteed to terminate in a finite amount of
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-- time and not hang forever.
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--
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2016-09-10 04:02:13 +02:00
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-- This guarantees that all Dhall programs can be safely reduced to a normal
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-- form where all functions have been evaluated. In fact, Dhall expressions can
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-- be evaluated even if all function arguments haven't been fully applied. For
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-- example, the following program is an anonymous function:
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2016-09-09 18:51:31 +02:00
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--
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-- > $ dhall
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-- > \(n : Bool) -> +10 * +10
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-- > <Ctrl-D>
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2016-09-17 05:16:46 +02:00
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-- > ∀(n : Bool) → Natural
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2016-09-09 18:51:31 +02:00
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-- >
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2016-09-17 05:16:46 +02:00
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-- > λ(n : Bool) → +100
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2016-09-09 18:51:31 +02:00
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--
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-- ... and even though the function is still missing the first argument named
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-- @n@ the compiler is smart enough to evaluate the body of the anonymous
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-- function ahead of time before the function has even been invoked.
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--
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-- Similarly, you can use this normalization process to remove indirection
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-- introduced by well-meaning software engineers over-architecting the
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-- configuration file.
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2016-09-09 18:17:25 +02:00
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module Dhall
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(
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-- * Input
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input
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-- * Types
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, Type
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2016-09-13 06:27:47 +02:00
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, Interpret(..)
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2016-09-09 18:51:31 +02:00
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, bool
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2016-09-09 18:17:25 +02:00
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, natural
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, integer
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, double
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, text
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2016-09-19 00:38:36 +02:00
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, maybe
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2016-09-09 18:17:25 +02:00
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, vector
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-- * Re-exports
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2016-09-19 00:24:54 +02:00
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, Text
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2016-09-09 18:17:25 +02:00
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, Vector
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, Generic
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) where
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2016-10-16 02:35:51 +02:00
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import Control.Applicative (empty, liftA2, (<|>))
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2016-09-09 18:17:25 +02:00
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import Control.Exception (Exception)
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2016-09-12 05:38:37 +02:00
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import Data.ByteString.Lazy (ByteString)
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2016-09-09 18:17:25 +02:00
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import Data.Text.Lazy (Text)
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import Data.Vector (Vector)
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2016-10-18 18:18:01 +02:00
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import Dhall.Core (Expr(..))
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import Dhall.TypeCheck (X)
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2016-09-09 18:17:25 +02:00
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import GHC.Generics
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import Numeric.Natural (Natural)
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2016-09-19 00:38:36 +02:00
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import Prelude hiding (maybe)
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2016-09-09 18:17:25 +02:00
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import qualified Control.Exception
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import qualified Data.Map
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import qualified Data.Text.Lazy
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2016-10-11 18:18:52 +02:00
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import qualified Data.Text.Lazy.Builder
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2016-09-19 00:38:36 +02:00
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import qualified Data.Vector
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2016-09-09 18:17:25 +02:00
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import qualified Dhall.Core
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import qualified Dhall.Import
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import qualified Dhall.Parser
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2016-10-18 03:34:51 +02:00
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import qualified Dhall.TypeCheck
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2016-09-09 18:17:25 +02:00
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import qualified GHC.Generics
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throws :: Exception e => Either e a -> IO a
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throws (Left e) = Control.Exception.throwIO e
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throws (Right r) = return r
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{-| Type-check and evaluate a Dhall program, decoding the result into Haskell
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The first argument determines the type of value that you decode:
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2016-09-10 04:02:13 +02:00
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>>> input integer "2"
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2
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2016-09-15 18:34:55 +02:00
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>>> input (vector double) "[ 1.0, 2.0 ] : List Bool"
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2016-09-10 04:02:13 +02:00
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[1.0,2.0]
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2016-09-09 18:17:25 +02:00
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Use `auto` to automatically select which type to decode based on the
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inferred return type:
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2016-09-10 04:02:13 +02:00
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>>> input auto "True" :: IO Bool
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True
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2016-09-09 18:17:25 +02:00
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-}
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input
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:: Type a
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-- ^ The type of value to decode from Dhall to Haskell
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2016-09-12 05:38:37 +02:00
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-> ByteString
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2016-09-09 18:17:25 +02:00
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-- ^ The Dhall program
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-> IO a
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-- ^ The decoded value in Haskell
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2016-09-12 05:38:37 +02:00
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input (Type {..}) bytes = do
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expr <- throws (Dhall.Parser.exprFromBytes bytes)
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2016-09-09 18:17:25 +02:00
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expr' <- Dhall.Import.load Nothing expr
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2016-10-18 03:34:51 +02:00
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typeExpr <- throws (Dhall.TypeCheck.typeOf (Annot expr' expected))
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2016-09-09 18:17:25 +02:00
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case extract (Dhall.Core.normalize expr') of
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Just x -> return x
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Nothing -> fail "input: malformed `Type`"
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2016-09-10 04:02:13 +02:00
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{-| A @(Type a)@ represents a way to marshal a value of type @\'a\'@ from Dhall
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2016-09-09 18:17:25 +02:00
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into Haskell
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You can produce `Type`s either explicitly:
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2016-09-15 18:03:12 +02:00
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> example :: Type (Vector Text)
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> example = vector text
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2016-09-09 18:17:25 +02:00
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... or implicitly using `auto`:
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2016-09-15 18:03:12 +02:00
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> example :: Type (Vector Text)
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2016-09-09 18:17:25 +02:00
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> example = auto
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You can consume `Type`s using the `input` function:
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> input :: Type a -> Text -> IO a
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-}
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data Type a = Type
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{ extract :: Expr X -> Maybe a
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, expected :: Expr X
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}
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deriving (Functor)
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{-| Decode a `Bool`
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>>> input bool "True"
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True
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-}
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bool :: Type Bool
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bool = Type {..}
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where
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extract (BoolLit b) = pure b
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extract _ = Nothing
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expected = Bool
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{-| Decode a `Natural`
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>>> input natural "+42"
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42
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-}
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natural :: Type Natural
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natural = Type {..}
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where
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extract (NaturalLit n) = pure n
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extract _ = empty
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expected = Natural
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{-| Decode an `Integer`
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>>> input integer "42"
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42
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-}
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integer :: Type Integer
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integer = Type {..}
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where
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extract (IntegerLit n) = pure n
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extract _ = empty
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expected = Integer
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{-| Decode a `Double`
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>>> input double "42.0"
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42.0
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-}
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double :: Type Double
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double = Type {..}
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where
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extract (DoubleLit n) = pure n
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extract _ = empty
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expected = Double
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{-| Decode `Text`
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>>> input text "\"Test\""
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"Test"
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-}
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text :: Type Text
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text = Type {..}
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where
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2016-10-11 18:18:52 +02:00
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extract (TextLit t) = pure (Data.Text.Lazy.Builder.toLazyText t)
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2016-09-09 18:17:25 +02:00
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extract _ = empty
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expected = Text
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2016-09-19 00:38:36 +02:00
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{-| Decode a `Maybe`
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>>> input (maybe integer) "[] : Maybe Integer"
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Nothing
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-}
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maybe :: Type a -> Type (Maybe a)
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maybe (Type extractIn expectedIn) = Type extractOut expectedOut
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where
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extractOut (MaybeLit _ es) = traverse extractIn es'
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where
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es' = if Data.Vector.null es then Nothing else Just (Data.Vector.head es)
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expectedOut = App Maybe expectedIn
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2016-09-09 18:17:25 +02:00
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{-| Decode a `Vector`
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2016-09-15 18:34:55 +02:00
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>>> input (vector integer) "[ 1, 2, 3 ] : List Integer"
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2016-09-09 18:17:25 +02:00
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[1,2,3]
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-}
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vector :: Type a -> Type (Vector a)
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vector (Type extractIn expectedIn) = Type extractOut expectedOut
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where
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extractOut (ListLit _ es) = traverse extractIn es
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2016-09-15 18:03:12 +02:00
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expectedOut = App List expectedIn
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2016-09-09 18:17:25 +02:00
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{-| Any value that implements `Interpret` can be automatically decoded based on
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the inferred return type of `input`
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2016-09-15 18:34:55 +02:00
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>>> input auto "[1, 2, 3 ] : List Integer" :: IO (Vector Integer)
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2016-09-09 18:17:25 +02:00
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[1,2,3]
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2016-09-10 04:02:13 +02:00
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This class auto-generates a default implementation for records that
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2016-09-13 06:27:47 +02:00
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implement `Generic`. This does not auto-generate an instance for sum types
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nor recursive types.
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2016-09-09 18:17:25 +02:00
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-}
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class Interpret a where
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auto :: Type a
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default auto :: (Generic a, GenericInterpret (Rep a)) => Type a
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auto = fmap GHC.Generics.to genericAuto
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instance Interpret Bool where
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auto = bool
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instance Interpret Natural where
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auto = natural
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instance Interpret Integer where
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auto = integer
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instance Interpret Double where
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auto = double
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instance Interpret Text where
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auto = text
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2016-09-19 00:38:36 +02:00
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instance Interpret a => Interpret (Maybe a) where
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auto = maybe auto
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2016-09-09 18:17:25 +02:00
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instance Interpret a => Interpret (Vector a) where
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auto = vector auto
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class GenericInterpret f where
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genericAuto :: Type (f a)
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2016-10-16 02:35:51 +02:00
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instance GenericInterpret f => GenericInterpret (M1 D d f) where
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genericAuto = fmap M1 genericAuto
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instance GenericInterpret V1 where
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genericAuto = Type {..}
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where
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extract _ = Nothing
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expected = Union Data.Map.empty
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instance (GenericInterpret f, GenericInterpret g) => GenericInterpret (f :+: g) where
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genericAuto = Type {..}
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where
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extract e = fmap L1 (extractL e) <|> fmap R1 (extractR e)
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expected = Union (Data.Map.union expectedL expectedR)
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Type extractL (Union expectedL) = genericAuto
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Type extractR (Union expectedR) = genericAuto
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instance (Constructor c, GenericInterpret f) => GenericInterpret (M1 C c f) where
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genericAuto = Type {..}
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where
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n :: M1 i c f a
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n = undefined
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name = Data.Text.Lazy.pack (conName n)
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extract (UnionLit name' e _)
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| name == name' = fmap M1 (extract' e)
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| otherwise = Nothing
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expected = Union (Data.Map.singleton name expected')
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Type extract' expected' = genericAuto
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2016-09-09 18:17:25 +02:00
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instance GenericInterpret U1 where
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genericAuto = Type {..}
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where
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extract _ = Just U1
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expected = Record (Data.Map.fromList [])
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instance (GenericInterpret f, GenericInterpret g) => GenericInterpret (f :*: g) where
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genericAuto = Type {..}
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where
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extract = liftA2 (liftA2 (:*:)) extractL extractR
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expected = Record (Data.Map.union ktsL ktsR)
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where
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Record ktsL = expectedL
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Record ktsR = expectedR
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Type extractL expectedL = genericAuto
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Type extractR expectedR = genericAuto
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instance (Selector s, Interpret a) => GenericInterpret (M1 S s (K1 i a)) where
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genericAuto = Type {..}
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where
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n :: M1 i s f a
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n = undefined
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extract (RecordLit m) = do
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case selName n of
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"" -> Nothing
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name -> do
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e <- Data.Map.lookup (Data.Text.Lazy.pack name) m
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fmap (M1 . K1) (extract' e)
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extract _ = Nothing
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expected = Record (Data.Map.fromList [(key, expected')])
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where
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key = Data.Text.Lazy.pack (selName n)
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Type extract' expected' = auto
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