[Haskell-cafe] Parsec monad transformer with IO?

Daniel Fischer daniel.is.fischer at web.de
Thu Mar 18 18:12:35 EDT 2010

Am Donnerstag 18 März 2010 22:34:48 schrieb Andrew Coppin:
> Job Vranish wrote:
> > Hoogle is a great tool for finding haskell functions:
> >
> > http://www.haskell.org/hoogle/
> >
> > You can punch in the type of a function you want and it will give you
> > a list of functions that might do what you need.
> > Generalizing the types a bit usually helps. Searching for either  m a
> > -> n m a   or   IO a -> m a   would give you 'lift' and 'liftIO' as
> > one of the top results.
> Is there a tool anywhere which can figure out how to construct a
> function with a specific type signature? Hoogle works if the thing you
> seek is a single function, but not so much if you need to throw several
> functions together.
> (For example, the signature "x -> [x -> y] -> [y]" can be implemented by
> \ x -> map ($ x), but this is initially non-obvious.)

does that, but it has serious limitations (it doesn't know [], and it 
doesn't accept recursive types, so you can't define a list-type yourself:

Djinn> data List a = Nil | Cons a (List a)
Error: Recursive types are not allowed: List
So, no luck with x -> [x -> y] -> [y].

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