Is `[ in ]` equivalent to `forall`?

Question

I noticed something in a snippet of code I was given:

var D: domain(2) dmapped Block(boundingBox=Space) = Space;
var A: [D] int;
[a in A] a = a.locale.id;

Is [a in A] equivalent to forall a in A a = a.locale.id?

Answer 1

For the most part, yes. In Chapel, [a in A] expr can be thought of as a shorthand for forall a in A do expr. However, there is a slight difference in that if A does not support parallel iteration, the forall form will generate a compile-time error whereas the [a in A] form will fall back to serial iteration.

With respect to the title of this question, note that this behavior is independent of whether or not A is distributed. For example, you could also write [i in 1..n] rather than forall i in 1..n do even though ranges like 1..n are never distributed in Chapel.

Array types in Chapel, like [D] real can similarly be read as "for all indices in D, allocate an element of type real."