[Haskell-beginners] Problems with IO actions

[Jason Dusek]

2009/07/01 Adam Bergmark <adam at edea.se>:
> * I'm still a little unsure of why mapM_, sequence and the
>   like are necessary, could someone please explain this, or
>   point me somewhere where I can read up on it?

  Say we have a list of `IO` actions (type `[IO a]`). We can't
  run lists directly; the runtime runs `IO` action (type `IO
  something`). It's easy to do the translation but we'd rather
  not do it all the time: here's an easy recursive definition of
  `sequence`:

    sequence ops             =  recurse [] ops
     where
      recurse acc [       ]  =  return $ reverse acc
      recurse acc (op:rest)  =  do
        result              <-  op
        sequence' (result:acc) rest

  The other operations are further conveniences that can be
  built up from `sequence`.

> * Why does this problem occur when there is only one action to
>   perform?

  Do you close the handle?

--
Jason Dusek