#7332: Kind-defaulting omitted leads to deeply obscure type error
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Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Oleg writes:Here is the simpified code to
reproduce the problem.
{{{
{-# LANGUAGE NoMonomorphismRestriction #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances, FlexibleContexts #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE OverloadedStrings #-}
module P where
import GHC.Exts( IsString(..) )
import Data.Monoid
newtype DC d = DC d
deriving (Show, Monoid)
instance IsString (DC String) where
fromString = DC
class Monoid acc => Build acc r where
type BuildR r :: * -- Result type
build :: (acc -> BuildR r) -> acc -> r
instance Monoid dc => Build dc (DC dx) where
type BuildR (DC dx) = DC dx
build tr acc = tr acc
instance (Build dc r, a ~ dc) => Build dc (a->r) where
type BuildR (a->r) = BuildR r
build tr acc s = build tr (acc `mappend` s)
-- The type is inferred
tspan :: (Monoid d, Build (DC d) r, BuildR r ~ DC d) => r
tspan = build (id :: DC d -> DC d) mempty
-- foo = tspan "aa"
-- foo1 = tspan (tspan "aa")
bar = tspan "aa" :: DC String
}}}
This compiles, but if I uncomment the definition `foo`, the compiler
complains
{{{
/tmp/p.hs:39:1:
Couldn't match type `[Char]' with `DC d'
When checking that `foo'
has the inferred type `forall t d a.
(IsString a, Monoid d, Build (DC d) (a -> t),
BuildR (a -> t) ~ DC d) =>
t'
Probable cause: the inferred type is ambiguous
}}}
However, the same code on GHC 7.4.1 type checks with no problem. The
compiler infers for foo:
{{{
foo :: (IsString (DC d), Monoid d, Build (DC d) t, BuildR t ~ DC d) => t
}}}
which is exactly as I would expect.
If you uncomment `foo1`, a much bigger error message emerges
{{{
/tmp/p.hs:41:1:
Could not deduce (BuildR t0 ~ DC d0)
from the context (IsString a,
Monoid d,
Monoid d1,
Build (DC d) (t1 -> t),
Build (DC d1) (a -> t1),
BuildR (a -> t1) ~ DC d1,
BuildR (t1 -> t) ~ DC d)
bound by the inferred type for `foo1':
(IsString a, Monoid d, Monoid d1, Build (DC d) (t1 -> t),
Build (DC d1) (a -> t1), BuildR (a -> t1) ~ DC d1,
BuildR (t1 -> t) ~ DC d) =>
t
at /tmp/p.hs:41:1-25
The type variables `t0', `d0' are ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Expected type: DC d0
Actual type: BuildR (a0 -> t0)
When checking that `foo1'
has the inferred type `forall t d t1 d1 a.
(IsString a, Monoid d, Monoid d1, Build (DC
d) (t1 -> t),
Build (DC d1) (a -> t1), BuildR (a -> t1) ~
DC d1,
BuildR (t1 -> t) ~ DC d) =>
t'
Probable cause: the inferred type is ambiguous
}}}
The error message indeed sounds like there is a problem: the type
variables `t0` and `d0` aren't mentioned anywhere else. However, GHC 7.4.1
does not have any problem with `foo1`. It accepts it and infers for it the
same type as for `foo`. Again, this is what I'd expect.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7332>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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