RE: Typechecker / OverloadedStrings question 7.8 vs. 7.10

[Simon Peyton Jones]

I think it’s because of the newly generalised Foldable stuff.  In 7.10, after 
huge discussion (https://ghc.haskell.org/trac/ghc/wiki/Prelude710) we have
elem :: (Eq a, Foldable t) => a -> t a -> Bool
all :: Foldable t => (a -> Bool) -> t a -> Bool

And there is no way to tell what ‘t’ you mean.  Lists?  Trees?  Who knows!

Simon

From: ghc-devs [mailto:ghc-devs-boun...@haskell.org] On Behalf Of Michael Karg
Sent: 30 July 2015 22:05
To: ghc-devs
Subject: Typechecker / OverloadedStrings question 7.8 vs. 7.10

Hi devs,
in the followin snippet:

{-# LANGUAGE OverloadedStrings #-}
import  Data.Char (isAlphaNum)
import  Data.ByteString.Char8 as BS (all)
main =
    print $ check str
  where
    check = BS.all (\x -> x `elem` valid || isAlphaNum x)  -- Line 7
    valid   = "$_-"                     -- :: String                      -- 
Line 8
    str      = "foo_bar123"
GHC 7.10 fails with the following errors (whereas 7.8 compiles without 
complaining):

ghc --make "Testcase.hs"
[1 of 1] Compiling Main             ( Testcase.hs, Testcase.o )
Testcase.hs:7:31:
    No instance for (Foldable t0) arising from a use of ‘elem’
    The type variable ‘t0’ is ambiguous
   (...)

Testcase.hs:8:15:
    No instance for (Data.String.IsString (t0 Char))
      arising from the literal ‘"$_-"’
    The type variable ‘t0’ is ambiguous
    (...)
Uncommenting the -- :: String type annotation (line 8) makes the snippet 
acceptable to the typechecker however.

So Foldable [] and  [Char] should be possible to infer, given the evidence of 
'isAlphaNum x', as obviously happens with GHC 7.8. My question is, how or why 
does the 7.10 typechecker behave differently? Is this intentional, or does this 
qualify for a trac ticket?
Thanks for looking into this,
Michael

PS: The ByteString part is just there since the snippet is taken out of one of 
my projects. The following (modified) code only typechecks on 7.10 with both 
type annotations uncommented:

{-# LANGUAGE OverloadedStrings #-}
import  Data.Char (isAlphaNum)
main =
    print $ check str
  where
    check   = all (\x -> x `elem` valid || isAlphaNum x)
    valid     = "$_-"               -- :: String
    str        = "foo_bar123"    -- :: String

The errors here are (1) no instances for Foldable and (2) no instances for 
IsString.


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