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Comment by lsb | Hacker News Reader

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lsb·

Yup, in Haskell it's as explicit as a <- for assignment, and = for what amounts to pattern equality.

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Iirc, it isn’t even assignment, it’s a monad extraction.

e.g.

a <- [2] …

gives a the value of 2, unwrapping the List monad (only works if the return value of the function is a List of some kind).

The above could also be written:

[2] >> (\a -> …)

It’s been a while since I’ve done much with haskell, so forgive me if I’ve mis-remembered something.

I accidentally downvoted this...

Aw :(

I think I’ll live with 1 less karma ;)

Compensated. Now nobody vote this parent up or down lest we disturb the gods.

I know that's what it does under the covers, but is there any reason not to think of it as assignment in your head? That's the mental construct I use in my extremely limited Haskell coding, anyway.

Because if you think of it as assignment, the fact that

a <- [2]

binds a to 2, rather than [2], makes no sense.

edit:

a better example (because it is more useful) would be:

a <- Just 2

which, again, binds a to 2, unwrapping the Maybe monad in the process. (I forget what happens if you try to do a <- Nothing)

As lanaer said, "<-" is really syntax sugar for ">>=" (bind), and could mean a variety of things (including assignment). "=" is binding a variable, as in "let", and is not symmetric. One could argue that a better name for "=" would be "->", as in "case".

(Comparison, which is symmetric, is "==".)

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