Mario authored 1 year ago and Mario committed 1 year ago

This squashed commit adds support for exportable named defaults, the accepted
GHC proposal at https://github.com/ghc-proposals/ghc-proposals/pull/409

The proposal extends the Haskell '98 declarations

    default (Int, Double)

which were implicitly always applying to Num class alone, to allow specifying
an arbitrary single-parameter class:

    default IsString (Text, String)

The effect of this declaration would be to eliminate the ambiguous type errors
around string literals when OverloadedStrings extension is active. The
declaration by itself has effect only in its module, so the proposal also adds
the ability to export class defaults:

    module MyModule (default IsIstring)

Once the language extension is published and established, we can consider using
it in base and other libraries.

See Note [Named default declarations] in GHC.Tc.Gen.Default
for implementation details.