On 30 Nov 2004, at 23:29, Ben Rudiak-Gould wrote:
Jules Bean wrote:
However, your problem *does* have a natural underlying monad, if you care to use it.
I may be confused, but I don't think it does. It seems like the OP wants a type like
data Perhaps a = Success a | Failure [Error]
[snip]
I don't see how to define (>>=) such that this will return the appropriate value. If a1 fails you must call p in order to collect additional errors, but there's no appropriate value of v1 that you can pass.
Yes, I see I spoke too quickly. Perhaps a = Success a | Failure [Error] is a monad, but the bind (>>=) operation doesn't do what the OP needs, since just like the bind for maybe, it will abort the computation when an error is returned. But you can define combinators in this monad which do what the OP wants (and you did one of them in your message), it's just that haskell's do notation doesn't work, since the do notation is a shorthand for >>=, and >>= is the wrong strategy for this problem. Jules