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

[Ivan Perez]

Correct me if I'm wrong here.

On Fri, 12 Apr 2019 at 05:21, Richard O'Keefe <raoknz at gmail.com> wrote:

> How does the right associativity of the short-circuiting
> Boolean operators in any way contradict the way that such operators work
> in other languages?  These operators are associative, so a && (b && c)
> necessarily has the same value and effects as (a && b) && c.
>

In pure Haskell, perhaps, but in other languages, I would say no.

In a language like C, I would expect that:
- a && b && c be represented in the AST as (a && b) && c
- The compiler optimizes the implementation of && to short circuit, which
is, in some way, using laziness.

This is not to say that they are right-associative; it's just a compiler
optimization.


>   It has never been the case that all operators in all programming
> languages were left associative.  For addition and subtraction it matters;
> you don't want a-b+c interpreted as a-(b+c), but not for || and not for
> &&.  My expectation is that these operators should be right associative.
>

I can't find any reference for logic itself and, because /\ is introduced
as associative from the start in propositional logic, it does not really
matter. However, my training as a kid in math and the exposure to how I
studied to solve (+) left to right (implicitly associating to the left)
would have led me to intuitively parse / parenthesize conjunctions with
multiple (&&) the same way unless instructed otherwise.

I think this portion of the Haskell Report is also relevant to this
intuition in the case of haskell programmers: "If no fixity declaration is
given for `op` then it defaults to highest precedence and left
associativity" (Section 4.4.2).

Cheers

Ivan
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