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Sometimes I see code with the following expression:

example = example' []

Or what is the different between these functions?

foldl
foldl'
ordonezalex
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XIA Yang
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4 Answers4

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' is simply another identifier character like any other. foldl' is a completely separate function from foldl; it could just as well be called strictFold. It's called that because it's closely related to foldl: it's a foldl where the accumulator is evaluated at every step, so that large thunks don't build up. For instance, foldl (+) 0 will overflow the stack on a large list, but foldl' (+) 0 won't.

In general, the suffixing of a ' means one of three things: foo' is either a helper definition made for the purpose of defining foo, a modified version of foo (that is, state, state', and state'' could be an initial state and two updated versions), or a strict version of foo.

ehird
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    You answered this question within like 3 mins of asking which may as well be the time taken to type it. I saw it and was like "hey a Haskell question that I can answer and to which no one has answered yet, bring the rep train", and I go typing the answer when a new answer appears from an 20k rep guy telling just what I was about to say. You, experts, should really leave some questions alone for guys like us... :) – Abdulsattar Mohammed Apr 28 '12 at 12:03
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    @CodingTales, answer it anyway! You might explain it slightly differently, and variety is always good! :) – huon Apr 28 '12 at 12:40
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    @CodingTales: The '20k brigade' tends to give rather succinct answers; I always like to explain a bit more and lead the asker to the answer a bit more gently, pointing out pitfalls and dead-end approaches. This particular question doesn't lend itself well to that, but I'm sure there'll be others! Also, you can set up email notifications for the haskell tag, and I'm sure having well-written explanations of the difference between `IO a` and `a`, the layout rule, and the monomorphism restriction at hand in a text file will help you beat them to the punch one day :-) – yatima2975 Apr 28 '12 at 14:08
  • @yatima2975 If you find a question asking about the difference between `IO a` and `a`, the layout rule, or the monomorphism restriction, please point those questioners at previous answers to similar questions rather than repeating the answer. – Daniel Wagner Apr 29 '12 at 02:22
4

Nothing, at least not in that context, since an apostrophe is a valid identifier character (see this answer for some more detail.)

It is a bit of a convention in the standard libraries that a strict version of certain functions is differentiated from the standard (lazy) version by an apostrophe, such as foldl' (strict) and foldl (lazy). This has nothing to do with the apostrophe being special though, it's just a convention.

Community
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huon
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2

In Haskell it is just another character to distinguish identifiers and the identifier is then called fold prime, but it is commonly used in the same way as it used in mathematics.

pmr
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0

There's a hackage module (I forget which one) that includes some kind of membership test (or some other predicate) in something. The test function is called "is":

foo x | x `is` whatever = ...

and for convenience the boolean negation of the function is also provided:

bar x | x `isn't` whatever = ...

I thought it was funny as heck when I first saw it. Apostrophe is just another character in the identifier.

none
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  • I haven't found the documentation of this feature yet, but I just came around this feature of haskell: In you case i guess the function is actually called ```is``` without the apostrophe and is intended to be used like that: ```is x whatever``` . but with the apostrophe it can be used like you used it! If anyone could supply any links on how this can be used in detail, that would be really nice! – Michael K. Apr 27 '15 at 09:45