Odditites regarding Any and typeclasses.

The typechecker seems to be unable to operate on contexts containing "naked" Any, for example:

x :: State [Any] ()
x = put []

    No instance for (MonadState [Any] (StateT [Any] Identity))
      arising from a use of ‘put’

A more minimal self-contained example would be:

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
import GHC.Prim (Any)
class Class a b where method :: a -> b
instance Class a a where method = id
x :: Any -> Any
x = method

    No instance for (Class Any Any) arising from a use of ‘method’

It's quite obvious why this error happens (Any is not a datatype), however it also feels like it shouldn't happen. All of the above code seems to work in 7.8, which is back when Any was a datatype.

Trac metadata

Trac field	Value
Version	7.10.2
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

Your second example compiles fine with HEAD (ghc-7.11.20151002).

closed

Confirmed that the first example works, too. I'm not sure what the fix is, though. I'm closing as fixed. If this is holding you up and you want the fix in 7.10.3, reopen and we can try to learn more.

Thanks for reporting!

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

changed milestone to %8.0.1

added Pnormal label