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{-# 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

-- 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

-- 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> 

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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