On Mon, Nov 30, 2009 at 03:02, Duncan Coutts
On Mon, 2009-11-30 at 06:08 +0000, Malcolm Wallace wrote:
However, if you really want to terminate the stream at the first error, and to reflect this in the type, then I guess you can define your own list type:
data ListThenError e a = Cons a (ListThenError e a) | Error e
Of course this has the disadvantage that then your consumer must change to use this type too.
I've been using this list type quite a lot recently. It's in the 'tar' package for example. It comes with variants of the standard functions foldl, foldr, unfoldr that take into account the error possibility.
At some point we should probably make a package to standardise and document this lazy error handling idiom.
Wow, this is perfect! I've extracted that type out into the "failable-list" library[1], with a few added instances for common classes (Monad, Applicative, Traversable, etc). [1] http://hackage.haskell.org/package/failable-list