[GHC] #12085: Premature defaulting and variable not in scope

19 May 2016

      #12085: Premature defaulting and variable not in scope
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  TypeApplications                   |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# Language RankNTypes, ScopedTypeVariables, TypeFamilies,
     TypeOperators, UnboxedTuples, UnicodeSyntax, ViewPatterns,
     QuasiQuotes, TypeInType, ApplicativeDo, TypeApplications,
     AllowAmbiguousTypes
 #-}

 import Data.Kind

 todo :: forall (a::Type). (Read a, Show a) => String
 todo = show @a (read @a "42")
 }}}

 there are two things I notice, even with `AllowAmbiguousTypes` the `a`
 gets defaulted prematurely

 {{{
 $ ghci -ignore-dot-ghci -fwarn-type-defaults /tmp/tl0z.hs
 GHCi, version 8.0.0.20160511: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/tl0z.hs, interpreted )
 Ok, modules loaded: Main.
 *Main> :t todo

 <interactive>:1:1: warning: [-Wtype-defaults]
     Defaulting the following constraints to type ‘()’
       (Read a0) arising from a use of ‘it’ at <interactive>:1:1
       (Show a0) arising from a use of ‘it’ at <interactive>:1:1
 foo :: String
 *Main>
 }}}

 instead of something like `todo :: forall a. (Read a, Show a) => String`,
 it can be applied to types

 {{{
 *Main> foo @Int
 "42"
 *Main> foo @Float
 "42.0"
 }}}

 ----

 The second thing is that if you want to add an argument independent of `a`

 {{{#!hs
 -- ghci> foo @Int False
 -- "100"
 -- ghci> foo @Float True
 -- "42.0"

 foo :: forall (a::Type). (Read a, Show a) => Bool -> String
 foo b = show @a (read @a (if b then "42" else "100"))
 }}}

 I found no way to define it as

 {{{#!hs
 -- ghci> foo False @Int
 -- "100"
 -- ghci> foo True @Float
 -- "42.0"
 foo :: Bool -> forall (a::Type). (Read a, Show a) => String
 foo b = show @a (read @a (if b then "42" else "100"))
 }}}

 to which GHC persists

 {{{
 $ ghci -ignore-dot-ghci -fwarn-type-defaults /tmp/tl0z.hs
 GHCi, version 8.0.0.20160511: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/tl0z.hs, interpreted )

 /tmp/tl0z.hs:10:15: error: Not in scope: type variable ‘a’

 /tmp/tl0z.hs:10:24: error: Not in scope: type variable ‘a’
 Failed, modules loaded: none.
 Prelude>
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12085
GHC http://www.haskell.org/ghc/
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