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 <simo...@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-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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