Re: [Haskell-cafe] Why are && and || right-associative?

Am 13.04.19 um 01:16 schrieb Viktor Dukhovni:

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On Apr 12, 2019, at 3:15 PM, Joachim Durchholz wrote:

What is the basis for this expectation? My expectation would be left associativity, just because I read stuff from left to right, and left associativity is what I get if I mentally parse from left to right. So I'm curious what thought process arrives at the opposite expectation.

Since (&&) short-circuits on first failure, and (||) short-circuits on first success, right associativity is more intuitive:

a && b && c == a && (b && c) a || b || c == a || (b || c)

as each can stop immediately when `a` is respectively False or True. This matches the fact that (&&) is naturally strict in its first argument and and lazy in the second, which works well with currying.

I guess my intuition is more based on math, where associativity is an irrelevant detail, then LR parsing, where left associativity requires less stack work. BTW I have a feeling that the LR parsing process is pretty natural: we read symbols from left to right, mentally combining them into groups as soon as possible so we can abstract away from individual symbols.

Also consider that, for example, in "ghci":

foldr (&&) True (replicate 100000000000 False)

returns immediately, while:

foldr (&&) True (replicate 100000000000 False)

hangs, which clearly shows that right folds are the more natural way to combine these boolean operators.

That's a Haskellism. Not that anything is wrong with that, of course :-)

And reading left to right, *is* IMHO right associative! You see:

a || ...

and immediately process a, then move on to what follows.

No, you see a || ... but you don't process anything, there's just a and ||. Then you see a || b ... but you still don't know what to do with that, because maybe the next operator has higher precedence (maybe &&, maybe even + or *). Then you see a || b || ... and now you can tick off the first three symbols: whatever || ... where somewhere back in your mind, whatever === a || b.

When reading an English sentence, you don't have to reach the last word before you can start to make sense of the preceding words, for left associativity, try German...

[ Oops! Never mind, perhaps that explains the difference in perspective. :-) :-) ]

Don't worry :-) Though I don't think that natural language processing works at any conscious level, so I don't think that that influence is too important. I German were the main influence, I would have to insist on postfix btw.