Hi,

Yes it's correct. You can check this with ghci:

f :: Integer -> Maybe Integer
f 5 = Nothing
f x = Just x

> let xs = [1..] :: [Integer]
> mapM f xs
Nothing  (it doesn't loop forever)
> :sprint xs
xs = 1 : 2 : 3 : 4 : 5 : _   (the tail after 5 is not evaluated)

I want to evaluate a function on a series of inputs:

f :: a -> Maybe b

then collect the results [b] if they are all Just, or terminate the computation immediately upon hitting Nothing. 

This is exactly what mapM does in the Maybe monad, correct?

In particular I want to make sure that it will not try to evaluate anything past the first 'Nothing' result as the efficiency of my design is based on that.

D

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