#14266: AllowAmbiguousTypes doesn't play well with default class methods -------------------------------------+------------------------------------- Reporter: chris-martin | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I like Richard's visible type pattern suggestion far more than trying to fiddle with `InstanceSigs`. (Well, except for the fact that we don't currently have visible type patterns. But I'll try not to be impatient.) If we do go down this route, I suppose we'd need to eta-expand //all// arguments of a default function when desugaring `DefaultSignatures` so that something like this would work: {{{#!hs class A t where f :: forall m. t m -> forall x. Monoid x -> m instance A [] where f @m tm @x = df @[] @m tm @x }}} Since I believe we'd need to visibly apply `@x` there in order to avoid being ensnared in ambiguity. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14266#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler