23 Jan
2009
23 Jan
'09
9:54 a.m.
On 23 Jan 2009, at 15:40, Kovacs David wrote:
Hello!
I have a function like this: http://rafb.net/p/8E66FI29.html, starting with evalState. The problem is with checkPredName cause it's return type is SemanticError what's not a monadic value but case waiting for m a (as the error message sais), but if I use return to checkPredName then ofc the pattern match will fail. How can I fix this?
Oops, sorry, I meant monad, because this monad unfortunately doesn't have it's associated Applicative instance (which is much nicer to work with). You can ofc create the standard applicative instance: instance Error e => Applicative (Either e) where pure = return (<*>) = ap Bob