Typeclass constraint should pick the OverloadedString type.

[GregWeber]

{-# LANGUAGE OverloadedStrings #-}
import Data.Text as T

class    NoDefault a      where noDefault :: a -> Text
instance NoDefault T.Text where noDefault = id

main = print (noDefault "Hello!")

default.hs:7:15:
    Ambiguous type variable `a0' in the constraints:
      (NoDefault a0) arising from a use of `noDefault'
                     at default.hs:7:15-23
      (Data.String.IsString a0) arising from the literal `"Hello!"'
                                at default.hs:7:25-32
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `print', namely `(noDefault "Hello!")'
    In the expression: print (noDefault "Hello!")
    In an equation for `main': main = print (noDefault "Hello!")

[michalt]

I don't really see a problem/bug here. The type is ambiguous and according to documentation the defaulting mechanism (IIRC to String) will not work here:

The standard defaulting rule (Haskell Report, Section 4.3.4) is extended thus: defaulting applies when all the unresolved constraints involve standard classes or IsString?; and at least one is a numeric class or IsString?.

So it's like writing:

module Main where

class Foo a where
  foo :: a -> ()

main = print (foo $ 0)

which results in

[1 of 1] Compiling Main             ( Test.hs, interpreted )

Test.hs:6:21:
    Ambiguous type variable `a0' in the constraints:
      (Num a0) arising from the literal `0' at Test.hs:6:21
      (Foo a0) arising from a use of `foo' at Test.hs:6:15-17
    Probable fix: add a type signature that fixes these type variable(s)
    In the second argument of `($)', namely `0'
    In the first argument of `print', namely `(foo $ 0)'
    In the expression: print (foo $ 0)
Failed, modules loaded: none.

Am I missing something?

[GregWeber]

Your example is very different for mine: it is missing a numeric instance. If the current behavior is properly documented now, consider this ticket a feature request.

[simonpj]

In that case, perhaps you can turn it into a feature request? That would mean

• reading the current specification (pointed to above),
• proposing a concrete change to the language specification, and
• reflecting on what consequences that might have for others.

Thanks.

[GregWeber]

ok, changed to a feature request. I am not smart enough to know what the consequences are for others: I was hoping that would be more obvious to those that work on the type system. I was just trying to write a program to be convenient for users and GHC would not allow me to, which makes me sad :(

I suspect this issue has not been brought up because generally one would write the typeclass for multiple IsString? instances. The undocumented defaulting capability, while putting a little extra burden on the users, seems generally a more powerful approach. So maybe after I can add documentation for defaulting I will close this.

[simonpj]

Maybe you can do the first two bullets even if you don't feel competent for the third?

[GregWeber]

ExtendedDefaultRules? is good but not fully covering my use cases of data types built of IsString?, so I am still very interested in proposing something here or improving ExtendedDefaultRules?. The following does not compile:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ExtendedDefaultRules #-}
{-# LANGUAGE FlexibleInstances #-}
module Default (noDefault) where
import Data.Text as T
default (T.Text)

class    NoDefault a        where noDefault :: a -> [Text]
instance NoDefault [T.Text] where noDefault = id

main = print (noDefault ["Hello!"])

[simonpj]

There is quite a bit of discussion on the ghc-users list about this. I'm going to park this ticket until someone emerges with a concrete proposal.

Simon

[GregWeber]

The discussion on the ghc-users thread veered off on another course. The ticket here still represents the original issues. In particular, I can't build up data structures with ExtendedDefaultRules?. defaulting is actually different than what I opened the original ticket for: should I open a different ticket?