Re: [Haskell-beginners] Maybe, Either

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 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 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

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