Type inference fails in connection with TypeFamilies, 'let', type equalities ...

The following example is a pretty involved use of type equality. It was accepted by GHC-8.0 and before, but GHC-8.2.0.20170505 cannot infer a type.

$ cat MonomorphismEquality.hs
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module MonomorphismEquality (run) where

newtype Value a = Value a

type family Repr (f :: * -> *) a :: *
type instance Repr f Int = f Int

class (Repr Value i ~ Value ir) => Native i ir where

instance Native Int Int where


fromInt :: (Native i ir) => i -> a
fromInt = undefined

apply :: (Int -> a -> a) -> a -> a
apply weight = id

run :: Float -> Float
run =
   let weight = \clip v -> fromInt clip * v
   in  apply weight

$ ghc-8.0.2 -Wall MonomorphismEquality.hs
[1 of 1] Compiling MonomorphismEquality ( MonomorphismEquality.hs, MonomorphismEquality.o )

MonomorphismEquality.hs:6:19: warning: [-Wunused-top-binds]
    Defined but not used: data constructor ‘Value’

MonomorphismEquality.hs:20:7: warning: [-Wunused-matches]
    Defined but not used: ‘weight’

$ ghc-8.2.0.20170505 -Wall MonomorphismEquality.hs
[1 of 1] Compiling MonomorphismEquality ( MonomorphismEquality.hs, MonomorphismEquality.o )

MonomorphismEquality.hs:24:28: error:
    • Ambiguous type variable ‘ir0’ arising from a use of ‘fromInt’
      prevents the constraint ‘(Native Int ir0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘ir0’ should be.
      These potential instance exist:
        instance Native Int Int -- Defined at MonomorphismEquality.hs:13:10
    • In the first argument of ‘(*)’, namely ‘fromInt clip’
      In the expression: fromInt clip * v
      In the expression: \ clip v -> fromInt clip * v
   |
24 |    let weight = \clip v -> fromInt clip * v
   |                            ^^^^^^^^^^^^

Trac metadata

Trac field	Value
Version	8.2.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import typechecker labels

Ah good. I know what is going on here. Fix coming.

mentioned in commit 43a31683

Thanks for the report.

Worth merging when convenient.

Trac metadata

Trac field	Value
Test case	- → indexed-types/should_compile/T13662

added 1 deleted label

changed milestone to %8.2.1

closed

Merged with b6f73dd7.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

Indeed, my problem is gone in ghc-8.2.0.20170517, thanks!

added Pnormal label