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

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Apr 19, 2019
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Add a better error message when decoding a bare None #895

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55 changes: 41 additions & 14 deletions dhall-json/src/Dhall/JSON.hs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,6 @@
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE OverloadedLists #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE OverloadedLists #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RecordWildCards #-}

{-| 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@
Expand Down Expand Up @@ -178,7 +177,6 @@ import Control.Exception (Exception, throwIO)
import Data.Aeson (Value(..), ToJSON(..))
import Data.Monoid ((<>), mempty)
import Data.Text (Text)
import Data.Typeable (Typeable)
import Dhall.Core (Expr)
import Dhall.TypeCheck (X)
import Dhall.Map (Map)
Expand All @@ -202,19 +200,43 @@ import qualified Options.Applicative
Because the majority of Dhall language features do not translate to JSON
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
show BareNone =
Data.Text.unpack $
_ERROR <> ": A bare ❰None❱ is not valid \n\
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❰None❱ on its own is not valid

\ \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\
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The “output is an absent value of that type” formulation is a bit confusing.

whose output is an `Optional` of that type

\ \n\
\For example: \n\
\ \n\
\ \n\
\ ┌─────────────────────────────────┐ This is a function whose result is an \n\
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`None` is a function

\ │ 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\
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`None Natural` is a valid

\ └─────────────────────────────────┘ absent ❰Natural❱ number in this case) \n\
\ \n\
\ \n\
\The conversion to JSON/YAML only translates the latter form to ❰null❱. "
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only translates the fully applied form (e.g. `None Natural`)

show (Unsupported e) =
Data.Text.unpack $
"" <> _ERROR <> ": Cannot translate to JSON \n\
\ \n\
\Explanation: Only primitive values, records, unions, ❰List❱s, and ❰Optional❱ \n\
\values can be translated from Dhall to JSON \n\
\ \n\
\The following Dhall expression could not be translated to JSON: \n\
\ \n\
\↳ " <> txt <> " "
_ERROR <> ": Cannot translate to JSON \n\
\ \n\
\Explanation: Only primitive values, records, unions, ❰List❱s, and ❰Optional❱ \n\
\values can be translated from Dhall to JSON \n\
\ \n\
\The following Dhall expression could not be translated to JSON: \n\
\ \n\
\↳ " <> txt <> " "
where
txt = Dhall.Core.pretty e

Expand Down Expand Up @@ -257,6 +279,11 @@ dhallToJSON e0 = loop (Dhall.Core.normalize e0)
return (toJSON a')
Dhall.Core.App Dhall.Core.None _ -> do
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 ->
case toOrderedList a of
[ ( "contents"
Expand Down