> This reads to me as “repeat the following block until this fails to be true”,
> the conditional binding in this case fails to be true if someCondition(value)
> isn’t true, so the loop ends. I think the key thing here is that the where
> clause is for the conditional binding and not the loop itself, so in this
> respect it behaves exactly like an if or guard statement. Meanwhile:
>
> for eachValue in theValues where someCondition(eachValue) { … }
>
> Reads as “for everything in theValues do the following if
> someCondition(eachValue) is also true”, in other words this loop always tries
> to visit every element of the sequence (a while loop has no implicit
> awareness of the sequence, it’s really just an if statement that runs over
> and over). In this case the where clause is part of the loop itself. There
> may be an argument that where should be renamed on for loops to better
> distinguish this, but once you consider that there’s no pattern or
> conditional binding here I think it makes a reasonable amount of sense.
The original sin here was in connecting the `where` clause to the for loop's
sequence expression, rather than its pattern. If `where` were positioned right
after the loop variable:
for eachValue where someCondition(eachValue) in theValues { … }
It would be much clearer that `where` constrains the values seen by the loop
body.
I'm not sure why the `where` clause was placed where it is. I suspect it has
something to do with the `where` clause potentially being more complex than the
sequence expression, but I was not in the room where it happened, so that's
idle speculation.
--
Brent Royal-Gordon
Architechies
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