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

Ivan Perez ivanperezdominguez at gmail.com
Fri Apr 12 02:26:44 UTC 2019

Could it be so that you can shortcut in the expected order (left to right)?

Left associative:
a && b && c = (a && b) && c which means going into a && b, which means
going into a, and if it is False, then going up in the expression tree.

If it is right associative:
a && b && c = a && (b && c), which means going into a, and if it is False,
you are done.

If you have a conjunction of n booleans, the complexity of evaluating this
expression is linear with respect to the position of the first False (in
the case of &&). In the left-associative case, it is linear in the number
of &&s.

Just a guess. But you got me interested now.

Does anyone have the real explanation?



On Thu, 11 Apr 2019 at 22:13, Richard Eisenberg <rae at richarde.dev> wrote:

> Hi café,
> Why are && and || in the Prelude right-associative? This contradicts my
> expectation and the way these work in other languages. That said, I can't
> think of any harm in it. This came up from a question asked by a student,
> and I have no idea why the design is this way.
> Thanks,
> Richard
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