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Add a better error message when decoding a bare None (#895)

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Fixes #492
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Gabriel Gonzalez 2019-04-19 09:30:05 -07:00 committed by GitHub
parent fcca883e00
commit 1f40b633d6
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1 changed files with 41 additions and 14 deletions
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55
dhall-json/src/Dhall/JSON.hs
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@ -1,7 +1,6 @@
{-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE OverloadedLists #-}
{-# LANGUAGE OverloadedLists #-} {-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE RecordWildCards #-}
{-| This library only exports a single `dhallToJSON` function for translating a {-| This library only exports a single `dhallToJSON` function for translating a
Dhall syntax tree to a JSON syntax tree (i.e. a `Value`) for the @aeson@ Dhall syntax tree to a JSON syntax tree (i.e. a `Value`) for the @aeson@
@ -178,7 +177,6 @@ import Control.Exception (Exception, throwIO)
import Data.Aeson (Value(..), ToJSON(..)) import Data.Aeson (Value(..), ToJSON(..))
import Data.Monoid ((<>), mempty) import Data.Monoid ((<>), mempty)
import Data.Text (Text) import Data.Text (Text)
import Data.Typeable (Typeable)
import Dhall.Core (Expr) import Dhall.Core (Expr)
import Dhall.TypeCheck (X) import Dhall.TypeCheck (X)
import Dhall.Map (Map) import Dhall.Map (Map)
@ -202,19 +200,43 @@ import qualified Options.Applicative
Because the majority of Dhall language features do not translate to JSON Because the majority of Dhall language features do not translate to JSON
this just returns the expression that failed this just returns the expression that failed
-} -}
data CompileError = Unsupported (Expr X X) deriving (Typeable) data CompileError
= Unsupported (Expr X X)
| BareNone
instance Show CompileError where instance Show CompileError where
show BareNone =
Data.Text.unpack $
_ERROR <> ": A bare ❰None❱ is not valid \n\
\ \n\
\Explanation: The conversion to JSON/YAML does not accept ❰None❱ in isolation as \n\
\a valid way to represent ❰null❱. In Dhall, ❰None❱ is a function whose input is \n\
\a type and whose output is an absent value of that type. \n\
\ \n\
\For example: \n\
\ \n\
\ \n\
\ ┌─────────────────────────────────┐ This is a function whose result is an \n\
\ │ None : ∀(a : Type) → Optional a │ ❰Optional❱ value, but the function \n\
\ └─────────────────────────────────┘ itself is not a valid ❰Optional❱ value \n\
\ \n\
\ \n\
\ ┌─────────────────────────────────┐ \n\
\ │ None Natural : Optional Natural │ This is a valid ❰Optional❱ value (an \n\
\ └─────────────────────────────────┘ absent ❰Natural❱ number in this case) \n\
\ \n\
\ \n\
\The conversion to JSON/YAML only translates the latter form to ❰null❱. "
show (Unsupported e) = show (Unsupported e) =
Data.Text.unpack $ Data.Text.unpack $
"" <> _ERROR <> ": Cannot translate to JSON \n\ _ERROR <> ": Cannot translate to JSON \n\
\ \n\ \ \n\
\Explanation: Only primitive values, records, unions, ❰List❱s, and ❰Optional❱ \n\ \Explanation: Only primitive values, records, unions, ❰List❱s, and ❰Optional❱ \n\
\values can be translated from Dhall to JSON \n\ \values can be translated from Dhall to JSON \n\
\ \n\ \ \n\
\The following Dhall expression could not be translated to JSON: \n\ \The following Dhall expression could not be translated to JSON: \n\
\ \n\ \ \n\
\↳ " <> txt <> " " \↳ " <> txt <> " "
where where
txt = Dhall.Core.pretty e txt = Dhall.Core.pretty e
@ -257,6 +279,11 @@ dhallToJSON e0 = loop (Dhall.Core.normalize e0)
return (toJSON a') return (toJSON a')
Dhall.Core.App Dhall.Core.None _ -> do Dhall.Core.App Dhall.Core.None _ -> do
return Data.Aeson.Null return Data.Aeson.Null
-- Provide a nicer error message for a common user mistake.
--
-- See: https://github.com/dhall-lang/dhall-lang/issues/492
Dhall.Core.None -> do
Left BareNone
Dhall.Core.RecordLit a -> Dhall.Core.RecordLit a ->
case toOrderedList a of case toOrderedList a of
[ ( "contents" [ ( "contents"