#13482: PartialTypeSignatures, AllowAmbiguousTypes and ScopedTypeVariables don't play nicely together -------------------------------------+------------------------------------- Reporter: dramforever | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- **Motivation**: I was playing with [http://lpaste.net/353915 something like this] when I found this great confusion. I was able to produce a minimal example. In each declaration/definition, the function body is the same, but the signature varies. I expect every single one of them to compile, resulting in the same type as `minimal4`. The actual results are shown in-line. I think this sufficiently shows that this behavior is a bug. (The line numbers in the following code aren't useful; I pasted them in after compiling) (The question marks are there because CMD wasn't able to display bullets, I think.) {{{#!hs {-# LANGUAGE PartialTypeSignatures #-} {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE KindSignatures #-} minimal1_noksig :: forall m. _ => Bool minimal1_noksig = (mempty :: m) == (mempty :: m) {- ambi.hs:17:30: error: ? Expected a type, but ‘m’ has kind ‘k0’ ? In an expression type signature: m In the first argument of ‘(==)’, namely ‘(mempty :: m)’ In the expression: (mempty :: m) == (mempty :: m) -} minimal1 :: forall (m :: *). _ => Bool minimal1 = (mempty :: m) == (mempty :: m) {- ambi.hs:11:1: error: ? Ambiguous type variable ‘m0’ prevents the constraint ‘(Monoid m0)’ from being solved. ? When checking that the inferred type minimal1 :: forall m. (Monoid m, Eq m) => Bool is as general as its (partial) signature minimal1 :: Bool -} minimal2 :: forall m. (Eq m, _) => Bool minimal2 = (mempty :: m) == (mempty :: m) {- ambi.hs:14:1: error: ? Could not deduce (Monoid m1) from the context: (Eq m, Monoid m) bound by the inferred type for ‘minimal2’: (Eq m, Monoid m) => Bool at ambi.hs:14:1-33 The type variable ‘m1’ is ambiguous ? When checking that the inferred type minimal2 :: forall m. (Monoid m, Eq m) => Bool is as general as its (partial) signature minimal2 :: forall m. (Eq m, Monoid m) => Bool -} minimal3 :: forall m. (Monoid m, _) => Bool minimal3 = (mempty :: m) == (mempty :: m) -- Compiles minimal4 :: forall m. (Monoid m, Eq m) => Bool minimal4 = (mempty :: m) == (mempty :: m) -- Compiles }}} The code was run in GHCi. GHC version is 8.0.2. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13482 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler