dhall-haskell/src/Dhall.hs

{-# LANGUAGE DefaultSignatures   #-}
{-# LANGUAGE DeriveFunctor       #-}
{-# LANGUAGE FlexibleInstances   #-}
{-# LANGUAGE FlexibleContexts    #-}
{-# LANGUAGE OverloadedStrings   #-}
{-# LANGUAGE RecordWildCards     #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators       #-}

-- | Dhall is a programming language specialized for configuration files.
--
-- The simplest possible way to use Dhall is to ignore the programming language
-- features and use it as a strongly typed configuration format.  For example,
-- suppose that you have the following configuration file:
-- 
-- > $ cat > config
-- > { foo = 1
-- > , bar = [ 3.0, 4.0, 5.0 : Double ]
-- > }
-- > <Ctrl-D>
-- 
-- You can read the above configuration file into Haskell using the following
-- code:
-- 
-- > {-# LANGUAGE DeriveGeneric     #-}
-- > {-# LANGUAGE OverloadedStrings #-}
-- > 
-- > import Dhall
-- > 
-- > data Example = Example { foo :: Integer , bar :: Vector Double }
-- >     deriving (Generic, Show)
-- > 
-- > instance Interpret Example
-- > 
-- > main :: IO ()
-- > main = do
-- >     x <- input auto "./config"
-- >     print (x :: Example)
-- 
-- The above program prints:
-- 
-- > $ ./example
-- > Example {foo = 1, bar = [3.0,4.0,5.0]}
--
-- In the above code, the `Example` Haskell type represents the schema for our
-- configuration file.  Suppose that we modify our configuration file to no
-- longer match the schema, like this:
--
-- > $ echo "1" > config
--
-- This then throws an exception when we try to load the configuration file:
--
-- > $ ./example
-- > example: 
-- > Expression: 1 : {{ bar : [ Double ], foo : Integer }}
-- > 
-- > Error: Expression's inferred type does not match annotated type
-- > 
-- > Explanation: You can annotate the type or kind of an expression like this:
-- > 
-- >     x : t  -- `x` is the expression and `t` is the annotated type or kind of `x`
-- > 
-- > Annotations are introduced in one of two ways:
-- > 
-- > * You can manually annotate expressions to declare the type or kind you expect
-- > * The interpreter also implicitly inserts a top-level type annotation
-- > 
-- > Annotations are optional because the compiler can infer the type of all
-- > expressions.  However, if you or the interpreter inserts an annotation and the
-- > inferred type or kind does not match the annotation then type-checking fails.
-- > 
-- > You or the interpreter annotated this expression:
-- > ↳ 1
-- > ... with this type or kind:
-- > ↳ {{ bar : [ Double ], foo : Integer }}
-- > ... but the inferred type of the expression is actually this type or kind:
-- > ↳ Integer
--
-- The Dhall programming language is a statically typed language and the
-- above error message is the output of the language's type-checker.  Every
-- expression we read into Haskell is type-checked against the expected schema.
--
-- The above error message says that the type-checker expected a record with
-- two fields: a field named @bar@ that is a `Vector` of `Double`s, and a
-- field named @foo@ that is an `Integer`.  However, the type-checker found an
-- expression whose inferred type was an `Integer`.  Since an `Integer` is not
-- the same thing as a record the type-checking step fails and Dhall does not
-- bother to marshal the configuration into Haskell.
--
-- Dhall is also a heavily restricted programming language.  For example, we can
-- define a configuration file that is an anonymous function:
--
-- > $ cat > makeBools
-- > \(n : Bool) ->
-- >         [ n && True, n && False, n || True, n || False : Bool ]
-- > <Ctrl-D>
--
-- You can read this as a function of one argument named @n@ of type `Bool`
-- that returns a `Vector` of `Bool`s.  Each element of the `Vector` depends
-- on the input argument.
--
-- This library comes with a command-line compiler named @dhall@ that you can
-- use to type-check configuration files and convert them to a normal form.  For
-- example, we can ask the compiler what the type of our @makeBools@ file
-- is:
--
-- > $ dhall typecheck < makeBools
-- > forall (n : Bool) → [ Bool ]
--
-- This says that @makeBools@ is a function of one argument named @n@ of type
-- `Bool` that returns a `Vector` of `Bool`s.
--
-- We can apply our file to a `Bool` argument as if it were an ordinary
-- function, like this:
--
-- > {-# LANGUAGE OverloadedStrings #-}
-- >
-- > import Dhall
-- >
-- > main :: IO ()
-- > main = do
-- >     x <- input auto "./makeBools True"
-- >     print (x :: Vector Bool)
--
-- This produces the following output:
--
-- > $ ./example
-- > [True,False,True,True]
--
-- Notice how we can decode into some types \"out-of-the-box\" without declaring
-- a Haskell record to store the output.  In the above example we marshalled
-- the result directly into a `Vector` of `Bool`s.  The instances for the
-- `Interpret` class list all types that are automatically supported.
--
-- We can also test functions directly on the command line using the @dhall@
-- compiler.  For example:
--
-- > $ dhall
-- > ./makeBools False
-- > <Ctrl-D>
-- > [ Bool ]
-- > 
-- > [ False, False, True, False : Bool ]
--
-- The @dhall@ compiler with no arguments produces two output lines:
--
-- * The first output line is the type of the result
-- * The second output line is the normal form of the expression that we input
--
-- In the above example the type of the result is a `Vector` of `Bool`s and the
-- normal form of the expression just evaluates all functions.
--
-- You can also use the Dhall compiler to evaluate expressions which have no
-- file references.  For example:
--
-- > $ dhall
-- > "Hello, " ++ "world!"
-- > <Ctrl-D>
-- > Text
-- > 
-- > "Hello, world!"
--
-- > $ dhall
-- > +10 * +10
-- > Natural
-- > 
-- > +100
--
-- Dhall is a very restricted programming language that only supports simple
-- operations.  For example, Dhall only support addition and subtraction on
-- `Natural` numbers (i.e. non-negative numbers), which are not the same type of
-- number as `Integer`s (which can be negative).  A `Natural` number is a number
-- prefixed with the @+@ symbol.  If you try to add or multiply two `Integer`s
-- (without the @+@ prefix) you will get a type error:
--
-- > $ dhall
-- > 2 + 2
-- > <Ctrl-D>
-- > dhall: 
-- > Expression: 2 + 2
-- > 
-- > Error: Cannot use `(+)` on a value that's not a `Natural`
-- > 
-- > Explanation: The `(+)` operator expects two arguments of type `Natural`
-- > 
-- > You provided this argument:
-- > 
-- >     2 + ...
-- > 
-- > ... whose type is not `Natural`.  The type is actually:
-- > ↳ Integer
-- > 
-- > An `Integer` is not the same thing as a `Natural` number.  They are distinct
-- > types: `Integer`s can be negative, but `Natural` numbers must be non-negative
-- > 
-- > You can prefix an `Integer` literal with a `+` to create a `Natural` literal
-- > 
-- > Example:
-- > 
-- >     +2 + ...
--
-- The Dhall language doesn't just type-check the final schema; the language
-- also ensures that every expression is internally consistent.  For example,
-- suppose that we call @./makeBools@ on a non-`Bool` argument:
--
--
-- > $ dhall
-- > ./makeBools "ABC"
-- > dhall: 
-- > dhall: 
-- > Expression: (\(n : Bool) → [ n && True, n && False, n || True, n || False : Bool ]) "ABC"
-- > 
-- > Error: Function applied to the wrong type or kind of argument
-- > 
-- > Explanation: Every function declares what type or kind of argument to accept
-- > 
-- >     \(x : Bool) → x   -- Anonymous function which only accepts `Bool` arguments
-- > 
-- >     let f (x : Bool) = x  -- Named function which only accepts `Bool` arguments
-- >     in  f True
-- > 
-- >     \(a : Type) → a   -- Anonymous function which only accepts `Type` arguments
-- > 
-- > You *cannot* apply a function to the wrong type or kind of argument:
-- > 
-- >     (\(x : Bool) → x) "AB"  -- "AB" is `Text`, but the function expects a `Bool`
-- > 
-- > You tried to invoke a function which expects an argument of type or kind:
-- > ↳ Bool
-- > ... on an argument of type or kind:
-- > ↳ Text
--
-- We get a type error saying that our function expects a `Bool` argument, but
-- we supplied an argument of type `Text` instead.
--
-- Our `input` function also doesn't need to reference any files at all:
--
-- >>> input auto "True && False" :: IO Bool
-- False
--
-- Reading from an external configuration file is just a special case of Dhall's
-- support for embedding files as expressions.  There's no limit to how many
-- files-as-expressions that you can nest this way.  For example, we can define
-- one file that is a Dhall expression that in turn depends on another file
-- which is also a Dhall expression:
--
-- > $ echo './bool1 && ./bool2' > both
-- > $ echo 'True'  > bool1
-- > $ echo 'False' > bool2
-- > $ dhall
-- > [ ./bool1 , ./bool2 , ./both : Bool ]
-- > <Ctrl-D>
-- > [ Bool ]
-- > 
-- > [ True, False, False : Bool ]
--
-- The only restriction is that the Dhall language will forbid cycles in these
-- file references:
--
-- > $ echo './bar' > foo
-- > $ echo './foo' > bar
-- > $ dhall < ./foo
-- > dhall: 
-- > ⤷ ./bar 
-- > ⤷ ./foo 
-- > Cyclic import: ./bar 
--
-- Dhall is a total programming language, which means that Dhall is not
-- Turing-complete and evaluation of every Dhall program is guaranteed to
-- eventually halt.  There is no upper bound on how long the program might take
-- to evaluate, but the program is guaranteed to terminate in a finite amount of
-- time and not hang forever.
--
-- This guarantees that all Dhall programs can be safely reduced to a normal
-- form where all functions have been evaluated.  In fact, Dhall expressions can
-- be evaluated even if all function arguments haven't been fully applied.  For
-- example, the following program is an anonymous function:
--
-- > $ dhall
-- > \(n : Bool) -> +10 * +10
-- > <Ctrl-D>
-- > forall (n : Bool) → Natural
-- > 
-- > \(n : Bool) → +100
--
-- ... and even though the function is still missing the first argument named
-- @n@ the compiler is smart enough to evaluate the body of the anonymous
-- function ahead of time before the function has even been invoked.
--
-- Similarly, you can use this normalization process to remove indirection
-- introduced by well-meaning software engineers over-architecting the
-- configuration file.

module Dhall
    (
    -- * Input
      input

    -- * Types
    , Type
    , bool
    , natural
    , integer
    , double
    , text
    , vector
    , pair2
    , pair3
    , Interpret(..)

    -- * Re-exports
    , Vector
    , Generic
    ) where

import Control.Applicative (empty, liftA2)
import Control.Exception (Exception)
import Data.ByteString.Lazy (ByteString)
import Data.Text.Lazy (Text)
import Data.Vector (Vector)
import Dhall.Core (Expr(..), X)
import GHC.Generics
import Numeric.Natural (Natural)

import qualified Control.Exception
import qualified Data.Map
import qualified Data.Text.Lazy
import qualified Dhall.Core
import qualified Dhall.Import
import qualified Dhall.Parser
import qualified GHC.Generics

throws :: Exception e => Either e a -> IO a
throws (Left  e) = Control.Exception.throwIO e
throws (Right r) = return r

{-| Type-check and evaluate a Dhall program, decoding the result into Haskell

    The first argument determines the type of value that you decode:

>>> input integer "2"
2
>>> input (vector double) "[ 1.0, 2.0 : Double ]"
[1.0,2.0]

    Use `auto` to automatically select which type to decode based on the
    inferred return type:

>>> input auto "True" :: IO Bool
True
-}
input
    :: Type a
    -- ^ The type of value to decode from Dhall to Haskell
    -> ByteString
    -- ^ The Dhall program
    -> IO a
    -- ^ The decoded value in Haskell
input (Type {..}) bytes = do
    expr     <- throws (Dhall.Parser.exprFromBytes bytes)
    expr'    <- Dhall.Import.load Nothing expr
    typeExpr <- throws (Dhall.Core.typeOf (Annot expr' expected))
    case extract (Dhall.Core.normalize expr') of
        Just x  -> return x
        Nothing -> fail "input: malformed `Type`"

{-| A @(Type a)@ represents a way to marshal a value of type @\'a\'@ from Dhall
    into Haskell

    You can produce `Type`s either explicitly:

> example :: Type (Double, Text)
> example = pair2 double text

    ... or implicitly using `auto`:

> example :: Type (Double, Text)
> example = auto

    You can consume `Type`s using the `input` function:

> input :: Type a -> Text -> IO a
-}
data Type a = Type
    { extract  :: Expr X -> Maybe a
    , expected :: Expr X
    }
    deriving (Functor)

{-| Decode a `Bool`

>>> input bool "True"
True
-}
bool :: Type Bool
bool = Type {..}
  where
    extract (BoolLit b) = pure b
    extract  _          = Nothing

    expected = Bool

{-| Decode a `Natural`

>>> input natural "+42"
42
-}
natural :: Type Natural
natural = Type {..}
  where
    extract (NaturalLit n) = pure n
    extract  _             = empty

    expected = Natural

{-| Decode an `Integer`

>>> input integer "42"
42
-}
integer :: Type Integer
integer = Type {..}
  where
    extract (IntegerLit n) = pure n
    extract  _             = empty

    expected = Integer

{-| Decode a `Double`

>>> input double "42.0"
42.0
-}
double :: Type Double
double = Type {..}
  where
    extract (DoubleLit n) = pure n
    extract  _            = empty

    expected = Double

{-| Decode `Text`

>>> input text "\"Test\""
"Test"
-}
text :: Type Text
text = Type {..}
  where
    extract (TextLit t) = pure t
    extract  _          = empty

    expected = Text

{-| Decode a `Vector`

>>> input (vector integer) "[ 1, 2, 3 : Integer ]"
[1,2,3]
-}
vector :: Type a -> Type (Vector a)
vector (Type extractIn expectedIn) = Type extractOut expectedOut
  where
    extractOut (ListLit _ es) = traverse extractIn es

    expectedOut = List expectedIn

{-| Decode a 2-tuple

>>> input (pair2 integer integer) "{ _1 = 1, _2 = 2 }"
(1,2)
-}
pair2 :: Type a -> Type b -> Type (a, b)
pair2
    (Type extractA expectedA)
    (Type extractB expectedB) = Type {..}
  where
    extract (RecordLit m) = do
        eA <- Data.Map.lookup "_1" m
        vA <- extractA eA
        eB <- Data.Map.lookup "_2" m
        vB <- extractB eB
        return (vA, vB)
    extract  _            = empty

    expected = Record (Data.Map.fromList kts)
      where
        kts =
            [ ("_1", expectedA)
            , ("_2", expectedB)
            ]

{-| Decode a 3-tuple

>>> input (pair3 integer integer integer) "{ _1 = 1, _2 = 2, _3 = 3 }"
(1,2,3)
-}
pair3 :: Type a -> Type b -> Type c -> Type (a, b, c)
pair3
    (Type extractA expectedA)
    (Type extractB expectedB)
    (Type extractC expectedC) = Type {..}
  where
    extract (RecordLit m) = do
        eA <- Data.Map.lookup "_1" m
        vA <- extractA eA
        eB <- Data.Map.lookup "_2" m
        vB <- extractB eB
        eC <- Data.Map.lookup "_3" m
        vC <- extractC eC
        return (vA, vB, vC)
    extract  _            = empty

    expected = Record (Data.Map.fromList kts)
      where
        kts =
            [ ("_1", expectedA)
            , ("_2", expectedB)
            , ("_3", expectedC)
            ]

{-| Any value that implements `Interpret` can be automatically decoded based on
    the inferred return type of `input`

>>> input auto "[1, 2, 3 : Integer]" :: IO (Vector Integer)
[1,2,3]

    This class auto-generates a default implementation for records that
    implement `Generic`.  This does auto-generate an instance for sum types or
    recursive types.
-}
class Interpret a where
    auto :: Type a
    default auto :: (Generic a, GenericInterpret (Rep a)) => Type a
    auto = fmap GHC.Generics.to genericAuto

instance Interpret Bool where
    auto = bool

instance Interpret Natural where
    auto = natural

instance Interpret Integer where
    auto = integer

instance Interpret Double where
    auto = double

instance Interpret Text where
    auto = text

instance Interpret a => Interpret (Vector a) where
    auto = vector auto

instance (Interpret a, Interpret b) => Interpret (a, b) where
    auto = pair2 auto auto

instance (Interpret a, Interpret b, Interpret c) => Interpret (a, b, c) where
    auto = pair3 auto auto auto

class GenericInterpret f where
    genericAuto :: Type (f a)

instance GenericInterpret U1 where
    genericAuto = Type {..}
      where
        extract _ = Just U1

        expected = Record (Data.Map.fromList [])

instance (GenericInterpret f, GenericInterpret g) => GenericInterpret (f :*: g) where
    genericAuto = Type {..}
      where
        extract = liftA2 (liftA2 (:*:)) extractL extractR

        expected = Record (Data.Map.union ktsL ktsR)
          where
            Record ktsL = expectedL
            Record ktsR = expectedR
        Type extractL expectedL = genericAuto
        Type extractR expectedR = genericAuto

instance GenericInterpret f => GenericInterpret (M1 C c f) where
    genericAuto = fmap M1 genericAuto

instance GenericInterpret f => GenericInterpret (M1 D c f) where
    genericAuto = fmap M1 genericAuto

instance (Selector s, Interpret a) => GenericInterpret (M1 S s (K1 i a)) where
    genericAuto = Type {..}
      where
        n :: M1 i s f a
        n = undefined

        extract (RecordLit m) = do
            case selName n of
                ""   -> Nothing
                name -> do
                    e <- Data.Map.lookup (Data.Text.Lazy.pack name) m
                    fmap (M1 . K1) (extract' e)
        extract  _            = Nothing

        expected = Record (Data.Map.fromList [(key, expected')])
          where
            key = Data.Text.Lazy.pack (selName n)

        Type extract' expected' = auto