#10361 closed bug (fixed)
DeriveAnyClass does not fill in associated type defaults
Reported by: | kosmikus | Owned by: | dreixel |
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Priority: | normal | Milestone: | 8.0.1 |
Component: | Compiler (Type checker) | Version: | 7.10.1 |
Keywords: | Generics, deriving | Cc: | vagarenko |
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | GHC rejects valid program | Test Case: | |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Rev(s): | Phab:D1283 | |
Wiki Page: |
Description
I would expect test1
and test2
below to typecheck.
This is a reduced test case from trying to use DeriveAnyClass
on the Generic
class of the generics-sop
package, which unfortunately fails due to this bug.
{-# LANGUAGE DeriveAnyClass, StandaloneDeriving, TypeFamilies #-} module Test where class C1 a where type T1 a type instance T1 a = Char class C2 a where -- equivalent to C1 type T2 a type instance T2 a = Char class C3 a where -- equivalent to C1, C2 type T3 a type instance T3 a = Char data A = B deriving C1 deriving instance C2 A instance C3 A -- fails -- test1 :: T1 A -- test1 = 'x' -- fails -- test2 :: T2 A -- test2 = 'x' -- succeeds test3 :: T3 A test3 = 'x'
Change History (11)
comment:1 Changed 3 years ago by
Component: | Compiler → Compiler (Type checker) |
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Type of failure: | None/Unknown → GHC rejects valid program |
comment:2 Changed 3 years ago by
Owner: | set to dreixel |
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comment:3 Changed 3 years ago by
I agree that this is a bug, yes. Not entirely sure when I will have time to fix it, though.
comment:4 Changed 3 years ago by
My case of this bug:
{-# LANGUAGE DefaultSignatures, TypeFamilies, TypeOperators, FlexibleContexts, DeriveGeneric, DeriveAnyClass #-} module Bug where import GHC.Generics --------------------------------------------------------------------- class Convert a where type Result a type instance Result a = GResult (Rep a) convert :: a -> Result a default convert :: (Generic a, GConvert (Rep a)) => a -> GResult (Rep a) convert x = gconvert (from x) instance Convert Float where type Result Float = Float convert = id instance Convert Int where type Result Int = Int convert = id --------------------------------------------------------------------- class GConvert f where type GResult f gconvert :: f p -> GResult f instance (Convert c) => GConvert (K1 i c) where type GResult (K1 i c) = Result c gconvert (K1 x) = convert x instance (GConvert f) => GConvert (M1 i t f) where type GResult (M1 i t f) = GResult f gconvert (M1 x) = gconvert x instance (GConvert f, GConvert g) => GConvert (f :*: g) where type GResult (f :*: g) = (GResult f, GResult g) gconvert (x :*: y) = (gconvert x, gconvert y) --------------------------------------------------------------------- -- This works: data Data1 = Data1 Int Float deriving (Generic) instance Convert Data1 val :: (Int, Float) val = convert $ Data1 0 0.0 -- This doesn't: data Data2 = Data2 Int Float deriving (Generic, Convert) -- Couldn't match type `Result Data2' with `(Int, Float)' -- Expected type: Data2 -> Result Data2 -- Actual type: Data2 -> GResult (Rep Data2) -- In the expression: Bug.$gdmconvert -- In an equation for `convert': convert = Bug.$gdmconvert -- When typechecking the code for `convert' -- in a derived instance for `Convert Data2': -- To see the code I am typechecking, use -ddump-deriv
comment:5 Changed 3 years ago by
Cc: | vagarenko added |
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comment:6 Changed 3 years ago by
Differential Rev(s): | → Phab:D1283 |
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comment:9 Changed 3 years ago by
Keywords: | Generics added |
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comment:10 Changed 3 years ago by
Milestone: | → 8.0.1 |
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comment:11 Changed 13 months ago by
Keywords: | deriving added |
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Pedro, this is your bag, right?