[Haskell-cafe] Alternative to newtypes or orphan instances

[Sean Leather]

On Mon, Aug 24, 2015 at 1:37 PM, Clinton Mead wrote:

> I see the problem now Sean. So in your example, is this correct:
>
> "To use the machinery of 'Show', I have to add and instance Show T. But
> that would be an orphan instance. So there is no way for T to use show
> machinery without adding an orphan instance (aside from modifying a module
> not under your control)?"
>

Correct.

Even newtype doesn't seem to help in your example. Show has all this
> machinery, but it seems completely unusable without adding an orphan
> instance or re-writing other people's modules. Is there no other way? This
> situation seems incredibly against re-usability, which I thought was
> Haskell's strength. Am I missing something?
>

Orphan instances are one of unfortunate pain points of Haskell, though they
are sometimes a necessary “evil.” Erik's first response describes a common
technique for publishing and sharing orphan instances. See, for example,
https://hackage.haskell.org/packages/search?terms=instances .

Regards,
Sean
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