Typechecker / OverloadedStrings question 7.8 vs. 7.10

[Michael Karg]

Hi Simon and all,

thanks for the quick response. I somehow suspected it was connected to
that...

The mildly surprising effect of adding/leaving out the OverloadedString
pragma however made me scratch my head a little, since the following code
(w/o pragma) does typecheck just fine (without annotating :: String):
import  Data.Char (isAlphaNum)
main =
    print $ check str
  where
    check   = all (\x -> x `elem` valid || isAlphaNum x)
    valid   = "$_-"             -- :: String
    str     = "foo_bar123"   -- :: String

I guess without that pragma, the string literals already imply t ~ [] for
Foldable t.

Thanks again for the answer, the behaviour I described is to be expected
then.
Michael



2015-07-30 23:24 GMT+02:00 Simon Peyton Jones <simonpj at microsoft.com>:

> 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-bounces at 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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