[CLJ-1124] for-as

Mon, 10 Dec 2012 23:33:43 -0600

A common pattern in programming is building up some data structure step by step:

In Python:

[code]
x = {0: 1}
for item in stuff:
x[item] = item * x.get(item - 1, 0)
[/code]
etc.

In an imperative for loop this is easy since we have easy access to the "current" data structure being built up.

I propose the addition of a function for-as similar to as-> except the value of the last loop iteration is bound to the name.

So we can write the above as:

[code]
(last (for-as [x {0 1}]
[item stuff]
(assoc x item (* item (get x (- item 1) 0)))))
[/code]

An (un-optimized) implementation might be something like:

[code]
(defmacro reduce-for [[res init] for-seq-exprs body-expr]
`(reduce #(%2 %1) ~init
(for ~for-seq-exprs
(fn [~res]
~body-expr))))
[/code]

Note: reduce-for does not return a seq, instead it returns the result of the last loop body iteration.

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