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[CLJ-1124] for-as Created: 10/Dec/12  Updated: 18/Apr/14  Resolved: 18/Apr/14

Status: Closed
Project: Clojure
Component/s: None
Affects Version/s: Release 1.5
Fix Version/s: None

Type: Enhancement Priority: Minor
Reporter: Yongqian Li Assignee: Unassigned
Resolution: Declined Votes: 0
Labels: None


 Description   

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

In Python:

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

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:

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

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

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

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



 Comments   
Comment by Kevin Downey [ 18/Apr/14 1:30 AM ]

this is not super clear to me, but it seems like a request for a `for` like macro for reduce.

(reduce (fn [x item] (assoc x item (* item (get x (- item 1) 0)))) {0 1} stuff)
Comment by Alex Miller [ 18/Apr/14 7:18 AM ]

This case seems adequately handled by reduce or loop/recur. I don't think the proposal is compelling enough to consider adding to core.

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