Unprincipled defaults

[David Feuer]

Whoa. I was far too confident that wouldn't work!

On Sep 2, 2016 12:39 PM, "Richard Eisenberg" <rae at cs.brynmawr.edu> wrote:

> I feel like I must be missing something, but what's wrong with
>
> class Semigroup1 f where
>   op :: f a -> f a -> f a
>
>   default op :: Monoid (f a) => f a -> f a -> f a
>   op = (<>)
>
>
> ? Does that do what you like?
>
> Richard
>
>
> On Sep 1, 2016, at 11:15 PM, David Feuer <david.feuer at gmail.com> wrote:
>
> On occasion, it can be useful to have default definitions that don't
> typecheck even with DefaultSignatures. It would be nice to be able to use
> them anyway. For example, if we have
>
> class Semigroup1 f where
>   op :: f a -> f a -> f a
>
> then we could, hypothetically, give a default definition for (<>):
>
>   default (<>) :: Semigroup1 f => f a -> f a -> f a
>   (<>) = op
>
> But we can't give a default definition
>
>   op = (<>)
>
> because there's no way to write its signature. However, for any F with
>
> instance Semigroup (F a) where ...
>
> the definition op = (<>) is perfectly fine.
>
> Would it be possible to offer a completely wild defaulting mechanism
> allowing *any expression* as a default, and only checking its type and
> compiling it when it's actually used?
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