Haskell: Calling a function multiple times within the same line

Question

If I have a function like the following

f x = if g x /= Nothing then g x else False

will g be called twice in f or does Haskell 'cache' the result of g x after the first instance for possible later use in the same line? I'm trying to optimise some code and I don't know if functions in the style of the one above are twice as computationally expensive as I want them to be.

Thanks in advance.

Also, whether things occur on the same line is irrelevant. It does affect the syntax, i.e. where a block starts and ends (which you can always write explicitly using { }), but other than that it does not affect anything. — luqui
– luqui, Commented Mar 2, 2013 at 18:21

Community · Accepted Answer · 2017-05-23 12:12:58Z

13

Haskell implementations do not memoize function calls.

Haskell compilers such as GHC do do common subexpression eliminiation but there are limitations.

If in doubt, share the result.

f x = if g' /= Nothing then g' else False
    where g' = g x

But you have a type error, since g' can't be both a boolean and a Maybe.

But better written as:

f x = case g x of
          Nothing -> ..
          Just _  -> ..

to compute and share the result in one branch only.

edited May 23, 2017 at 12:12

CommunityBot

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answered Mar 1, 2013 at 23:08

Don Stewart

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3 Comments

Robincognito Over a year ago

Thank you. That explains things perfectly. And yes, I was bit hasty in writing that example function.

kosmikus Over a year ago

How do you know that g' cannot be both a Boolean and a Maybe? Just for example, g could be read, and the original function would typecheck ...

Don Stewart Over a year ago

Ok, it could be instances of an overloaded g :). But \f g x -> if g x /= Nothing then g x else False isn't going to fly.

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