* Allow instances that require `Fail` to be empty

A class instance declaration that has `Prim.TypeError.Fail` as a constraint

will never be used. In light of this, such instances are now allowed to have

empty bodies even if the class has members.

(Such instances are still allowed to declare all of their members, and it is

still an error to specify some but not all members.)

import Control.Monad (unless)

-- Instance declarations with a Fail constraint are unreachable code, so

-- we allow them to be empty.

let unreachable = any ((C.Fail ==) . constraintClass) deps && null decls

unless unreachable $

case filter (\(ident, _) -> not $ S.member ident declaredMembers) memberTypes of

hd : tl -> throwError . errorMessage' ss $ MissingClassMember (hd NEL.:| tl)

[] -> pure ()

-- Create values for the type instance members

members <- zip (map typeClassMemberName decls) <$> traverse (memberToValue memberTypes) decls

-- Create the type of the dictionary

-- The type is a record type, but depending on type instance dependencies, may be constrained.

-- The dictionary itself is a record literal (unless unreachable, in which case it's undefined).

superclassesDicts <- for typeClassSuperclasses $ \(Constraint _ superclass _ suTyArgs _) -> do

let tyArgs = map (replaceAllTypeVars (zip (map fst typeClassArguments) tys)) suTyArgs

pure $ Abs (VarBinder ss UnusedIdent) (DeferredDictionary superclass tyArgs)

let superclasses = superClassDictionaryNames typeClassSuperclasses `zip` superclassesDicts

let props = Literal ss $ ObjectLiteral $ map (first mkString) (members ++ superclasses)

dictTy = foldl srcTypeApp (srcTypeConstructor (fmap (coerceProperName . dictTypeName) className)) tys

constrainedTy = quantify (foldr srcConstrainedType dictTy deps)

dict = App (Constructor ss (fmap (coerceProperName . dictTypeName) className)) props

mkTV = if unreachable then TypedValue False (Var nullSourceSpan C.I_undefined) else TypedValue True dict

result = ValueDecl sa name Private [] [MkUnguarded (mkTV constrainedTy)]

return result

Error found:

at tests/purs/failing/4483.purs:10:1 - 11:24 (line 10, column 1 - line 11, column 24)

The following type class members have not been implemented:

bar :: Int -> Int

in type class instance

 

 Main.Foo Int

 

See https://github.com/purescript/documentation/blob/master/errors/MissingClassMember.md for more information,

or to contribute content related to this error.

module Main where

import Prim.TypeError

class Foo t where

foo :: t -> String

bar :: Int -> t

instance fooInt :: Fail (Text "can't use this") => Foo Int where

foo _ = "unreachable"

-- bar is missing; you can get away with an empty instance here but not a

-- half-implemented one

module Main where

import Effect.Console (log)

import Prim.TypeError

class Foo t where

foo :: t -> String

bar :: Int -> t

instance fooInt :: Fail (Text "can't use this") => Foo Int

main = log "Done"