I often times have to write a lookup function that returns its value into any monad instead of just Maybe. For example:

mLookup :: (Eq k, Monad m) => k -> [(k, v)] -> m v
mLookup k pairs = case lookup k pairs of
Nothing -> fail "mLookup: nothing found"
Just v -> return v

Hope that helps.

On Sat, Sep 12, 2009 at 6:51 PM, Yusaku Hashimoto <nonowarn@gmail.com> wrote:

I think there is no such function in standard library.

But you can also do the same by `maybe (Left err) Right (lookup key
assoc)` without defining eitherLookup.

HTH
-nwn

On Sun, Sep 13, 2009 at 12:14 AM, Michael Mossey <mpm@alumni.caltech.edu> wrote:
> I want to use 'lookup' inside an Either String monad. So I want to write
> something like
>
> eitherLookup :: Eq a => String -> a -> [(a,b)] -> Either String b
> eitherLookup s x ps = case lookup x ps of
> Just y -> Right y
> Nothing -> Left s
>
> Is there such a function existing?
>
> Thanks,
> Mike
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners