[Haskell-cafe] Re: Scrap your rolls/unrolls

Max Bolingbroke batterseapower at hotmail.com
Fri Oct 22 07:24:37 EDT 2010

Forgot to reply to list

On 22 October 2010 12:14, Dan Doel <dan.doel at gmail.com> wrote:
> Another solution, though, is SHE. With it, you can write:
>  data ListF a r = NilF | ConsF a r
>  newtype List a = Roll (ListF a (List a))
>  pattern Nil       = Roll NilF
>  pattern Cons x xs = Roll (ConsF x xs)
> And not worry about Rolls anymore.

Ah yes, pattern synonyms. This solution is somewhat unsatisfying
because you will also need some smart constructors:

nil = Roll NilF
cons x xs = Roll (ConsF x xs)

Now the names of the smart constructors for building the data type are
not the same as the things you pattern match on, which is a slightly
annoying asymmetry. It's certainly less complicated than the
TypeFamilies based solution though!


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