Re: Typechecker / OverloadedStrings question 7.8 vs. 7.10

Hello, 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 -Iavor On Mon, Aug 3, 2015 at 9:47 AM, Brandon Allbery wrote:

On Mon, Aug 3, 2015 at 12:45 PM, Daniel Bergey 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@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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