In search of: [a->b] -> a -> [b]

[Derek Elkins]

On Thu, 19 Jun 2003 18:05:11 +0200
Christian Sievers <sievers@math2.nat.tu-bs.de> wrote:

> Derek Elkins wrote:
> 
> > >      flist :: [a->b] -> a -> [b]
> > >      flist fs a = map (flip ($) a) fs
> > or much nicer (IMO) 
> >   flist fs a = map ($ a) fs 
> 
> This is a case where I'd prefer a list comprehension:
> 
> flist fs a = [ f a | f <- fs ]
> 
> (and this could be a monad comprehension, if Haskell still had
> them...)

I don't think Haskell ever had them (I'd have to check).
Gofer did. Anyways, do-notation is about as expressive as monad
comprehensions. With do-notation you have to add guards explicitly, but
you don't have to end with a return and you don't need to bind
variables, e.g. I believe [() | _ <- putStrLn "foo"] would be necessary
(well, not in this case, but the general one).

> > the generalized solution being simply,
> > f mf x = do
> >     f <- mf
> >     return (f x)
> 
> Or just replace map by fmap in your flist from above.

That generalizes it, but in a different way since (in Haskell) Monad
isn't a subclass of Functor...