#13785: Cannot pacify -Wmonomorphism-restriction with nested pattern bindings -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I originally observed this issue in the `parsers` library. Here's a minimal example: {{{#!hs {-# LANGUAGE ScopedTypeVariables #-} {-# OPTIONS_GHC -Wmonomorphism-restriction #-} module Bug where class Monad m => C m where c :: (m Char, m Char) foo :: forall m. C m => m Char foo = bar >> baz where (bar, baz) = c }}} If you compile this with a GHC that supports `-Wmonomorphism-restriction` (GHC 8.0.1 or later), it'll rightfully give this warning: {{{ [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:11:5: warning: [-Wmonomorphism-restriction] • The Monomorphism Restriction applies to the bindings for ‘bar’, ‘baz’ Consider giving a type signature for these binders • In an equation for ‘foo’: foo = bar >> baz where (bar, baz) = c | 11 | (bar, baz) = c | ^^^^^^^^^^^^^^ }}} Naturally, I tried to squelch this warning by adding type signatures for `bar` and `baz`: {{{#!hs foo :: forall m. C m => m Char foo = bar >> baz where bar, baz :: m Char (bar, baz) = c }}} But GHC //still// warns! {{{ [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:12:5: warning: [-Wmonomorphism-restriction] • The Monomorphism Restriction applies to the bindings for ‘bar’, ‘baz’ Consider giving a type signature for these binders • In an equation for ‘foo’: foo = bar >> baz where bar, baz :: m Char (bar, baz) = c | 12 | (bar, baz) = c | ^^^^^^^^^^^^^^ }}} And to make things even more absurd, GHC points out a chunk of code which //has// type signatures :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13785 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler