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[CLJ-1217] for consumes sequence argument more eagerly than necessary Created: 14/Jun/13  Updated: 05/Feb/14

Status: Open
Project: Clojure
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Enhancement Priority: Minor
Reporter: Alan Malloy Assignee: Unassigned
Resolution: Unresolved Votes: 0
Labels: lazy

Attachments: Text File 0001-Don-t-realize-seq-exprs-in-for-unless-necessary.patch    
Patch: Code

 Description   

In a call like (do (for [x (do (println "realized") nil)] x) nil), no elements of the for comprehension are ever requested, and so it is not actually necessary to evaluate the inner do-block. However, this expression causes "realized" to be printed, because the first sequence-expression in for is evaluated even if no items are ever requested from the output lazy-seq.

It's not documented whether this is intended or unintentional, but I was surprised by this behavior, and a brief unscientific survey on #clojure suggests that other users, even "old hands" who've been using clojure for years, don't expect this either.

I've attached a patch that wraps the problematic expression in a lazy-seq call. This is not quite ideal, because it means that the first iteration is "lazied" twice, as in ((fn step [s] (lazy-seq ...)) (lazy-seq xs)), but a change to make this not happen would be much broader in scope, and this seemed the least dangerous.


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