Typechecker / OverloadedStrings question 7.8 vs. 7.10

Iavor Diatchki iavor.diatchki at gmail.com
Mon Aug 3 17:15:05 UTC 2015


what Reid says is exactly right---the issue is not really about what
instances are present, the problem is that GHC can't determine how to
instantiate `t0`.
Perhaps a more direct way to describe this is as follows:

Failed to infer type `t0`
  while solving constraint `Data.String.IsString (t0 Char)`
  arising from the use of:
    elem :: a -> t0 a -> Bool


On Mon, Aug 3, 2015 at 9:47 AM, Brandon Allbery <allbery.b at gmail.com> wrote:

> On Mon, Aug 3, 2015 at 12:45 PM, Daniel Bergey <bergey at teallabs.org>
> wrote:
>> I thought GHC would infer the type when only one instance is in scope,
>> at least in some cases, like IsString.  But I could well be wrong about
>> that.
> Typeclasses are open-world; this is not a safe assumption, since instances
> are global and an instance added elsewhere at some point in the future
> could therefore break your program.
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
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