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2374: Add error message for ambiguous type variables in inferred contexts (purescript#2410)
* 2374: Add error message for ambiguous type variables in inferred contexts * 2323 add contribution entry. remove Ident from AmbiguousTypeVariables since it's redundant with the enclosing compiler error's ident * 2323 update error message * 2374 multiline error message and print type, not type atom
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‎CONTRIBUTORS.md‎

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@@ -83,6 +83,7 @@ This file lists the contributors to the PureScript compiler project, and the ter
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- [@vkorablin](https://github.com/vkorablin) (Vladimir Korablin) - My existing contributions and all future contributions until further notice are Copyright Vladimir Korablin, and are licensed to the owners and users of the PureScript compiler project under the terms of the MIT license.
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- [@zudov](https://github.com/zudov) (Konstantin Zudov) My existing contributions and all future contributions until further notice are Copyright Konstantin Zudov, and are licensed to the owners and users of the PureScript compiler project under the terms of the [MIT license](http://opensource.org/licenses/MIT).
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- [@brandonhamilton](https://github.com/brandonhamilton) (Brandon Hamilton) My existing contributions and all future contributions until further notice are Copyright Brandon Hamilton, and are licensed to the owners and users of the PureScript compiler project under the terms of the [MIT license](http://opensource.org/licenses/MIT).
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- [@bbqbaron](https://github.com/bbqbaron) (Eric Loren) My existing contributions and all future contributions until further notice are Copyright Eric Loren, and are licensed to the owners and users of the PureScript compiler project under the terms of the [MIT license](http://opensource.org/licenses/MIT).
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### Companies
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‎examples/failing/ConstraintFailure.purs‎

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-- @shouldFailWith NoInstanceFound
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module Main where
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import Prelude
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data Foo = Bar
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spin :: forall a. a -> Foo
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spin x = Bar
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main = show <<< spin
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‎examples/failing/ConstraintInference.purs‎

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-- @shouldFailWith NoInstanceFound
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-- @shouldFailWith AmbiguousTypeVariables
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module Main where
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‎src/Language/PureScript/AST/Declarations.hs‎

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@@ -83,6 +83,7 @@ data SimpleErrorMessage
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| ConstrainedTypeUnified Type Type
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| OverlappingInstances (Qualified (ProperName 'ClassName)) [Type] [Qualified Ident]
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| NoInstanceFound Constraint
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| AmbiguousTypeVariables Type Constraint
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| UnknownClass (Qualified (ProperName 'ClassName))
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| PossiblyInfiniteInstance (Qualified (ProperName 'ClassName)) [Type]
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| CannotDerive (Qualified (ProperName 'ClassName)) [Type]

‎src/Language/PureScript/Errors.hs‎

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@@ -118,6 +118,7 @@ errorCode em = case unwrapErrorMessage em of
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ConstrainedTypeUnified{} -> "ConstrainedTypeUnified"
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OverlappingInstances{} -> "OverlappingInstances"
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NoInstanceFound{} -> "NoInstanceFound"
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AmbiguousTypeVariables{} -> "AmbiguousTypeVariables"
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UnknownClass{} -> "UnknownClass"
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PossiblyInfiniteInstance{} -> "PossiblyInfiniteInstance"
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CannotDerive{} -> "CannotDerive"
@@ -261,6 +262,7 @@ onTypesInErrorMessageM f (ErrorMessage hints simple) = ErrorMessage <$> traverse
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gSimple (ExprDoesNotHaveType e t) = ExprDoesNotHaveType e <$> f t
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gSimple (InvalidInstanceHead t) = InvalidInstanceHead <$> f t
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gSimple (NoInstanceFound con) = NoInstanceFound <$> overConstraintArgs (traverse f) con
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gSimple (AmbiguousTypeVariables t con) = AmbiguousTypeVariables <$> (f t) <*> pure con
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gSimple (OverlappingInstances cl ts insts) = OverlappingInstances cl <$> traverse f ts <*> pure insts
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gSimple (PossiblyInfiniteInstance cl ts) = PossiblyInfiniteInstance cl <$> traverse f ts
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gSimple (CannotDerive cl ts) = CannotDerive cl <$> traverse f ts
@@ -636,6 +638,11 @@ prettyPrintSingleError (PPEOptions codeColor full level showWiki) e = flip evalS
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where
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go TUnknown{} = True
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go _ = False
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renderSimpleErrorMessage (AmbiguousTypeVariables t _) =
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paras [ line "The inferred type"
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, indent $ line $ markCode $ prettyPrintType t
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, line "has type variables which are not mentioned in the body of the type. Consider adding a type annotation."
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]
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renderSimpleErrorMessage (PossiblyInfiniteInstance nm ts) =
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paras [ line "Type class instance for"
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, markCodeBox $ indent $ Box.hsep 1 Box.left

‎src/Language/PureScript/TypeChecker/Types.hs‎

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@@ -111,7 +111,7 @@ typesOf bindingGroupType moduleName vals = withFreshSubstitution $ do
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let solved = foldMap (S.fromList . fdDetermined) typeClassDependencies
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let constraintTypeVars = nub . foldMap (unknownsInType . fst) . filter ((`notElem` solved) . snd) $ zip (constraintArgs con) [0..]
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when (any (`notElem` unsolvedTypeVars) constraintTypeVars) $ do
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throwError . onErrorMessages (replaceTypes currentSubst) . errorMessage $ NoInstanceFound con
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throwError . onErrorMessages (replaceTypes currentSubst) . errorMessage $ AmbiguousTypeVariables generalized con
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-- Check skolem variables did not escape their scope
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skolemEscapeCheck val'

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