#12507: Can't deduce implicit parameter -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- I tried running a [https://gist.github.com/llelf/6c01ded225877914f38a gist] {{{#!hs {-# LANGUAGE GADTs, ConstraintKinds, Rank2Types, ImplicitParams #-} data Rec fields where Rec :: fields => Rec fields infixr 1 ? (?) :: Rec fields -> (fields => r) -> r Rec ? e = e record :: Rec (?a :: Int, ?b :: String) record = Rec where ?a=42 ?b="hey" access :: Int access = record ? ?a }}} and got {{{ $ ghci -ignore-dot-ghci tCHB.hs GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( tCHB.hs, interpreted ) tCHB.hs:15:19: error: • Could not deduce: ?a::Int arising from a use of implicit parameter ‘?a’ from the context: (?a::Int, ?b::String) bound by a type expected by the context: (?a::Int, ?b::String) => Int at tCHB.hs:15:10-20 • In the second argument of ‘(?)’, namely ‘?a’ In the expression: record ? ?a In an equation for ‘access’: access = record ? ?a Failed, modules loaded: none. Prelude> }}} I would certainly expect the context `(?a::Int, ?b::String)` to imply `?a::Int`? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12507 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler