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Comment by joechung | Hacker News Reader

"He hadn’t quite internalized left-hand side vs. right-hand side." This is noteworthy.

In mathematics, equality is symmetric (a=b is the same as b=a).

In programming, variable assignment is asymmetric (a=b is not the same as b=a).

Some programming languages recognize this but get the change wrong by changing the equality operator to == or === and always preferring assignment to be =. Some get it right, e.g., := as assignment.

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Some get it right, e.g., := as assignment.

This is a tradeoff of one hard day in your teens ("But but but equals should work both ways! head explodes") and thirty seconds every time you pick up a new language ("Assignment operator is equals? Got it.") versus typing more several dozen or hundred times a day every day for the rest of your professional life.

How about just making assignment the colon character?

kinda like YAML? www.yaml.org

Or getting rid of assignment altogether?

The traditional solution is ←.

Yes, but don't overlook the "Let x =" syntax of mathematics. Programming has just dropped the "let". So, really there is a choice to be made about how to resolve that ambiguity in a language parser. Visual Basic has the nice case of A = B = C setting A equal to the boolean expression B = C.

Old versions of BASIC used LET for assignment, but Microsoft BASIC and other BASICs of that era dropped it as being unnecessary.

Let in mathematics doesn't really work that way. It's more general.

"Let <boolean expression>" means that the world gets set up so that the boolean expression is true.

Examples:

Let x=4

Let epsilon>0

Let a in A

There's very interesting work on providing an iteration construct like this in a programming language.

http://www.cs.cornell.edu/Projects/jmatch/

Not completely, Lisp uses let for variable bindings...

  (let ((x 5)
        (y 10))
    (+ x y))

  => 15

I don't know where I picked this up, but I use asymmetric whitespace around the assignment operator: a= b. It provides a subtle hint that assignment is not equality.

You need to sit down with your text editor and see if you can teach it a few tricks.

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

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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