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[CLJ-1316] for doesn't support :let binding as its first seq-expr Created: 30/Dec/13  Updated: 28/Dec/14

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

Type: Enhancement Priority: Minor
Reporter: Jay Fields Assignee: Unassigned
Resolution: Unresolved Votes: 1
Labels: None

jvm clojure

user> (for [y [2 3 4] 
            :let [x 1]]
        [x y])
([1 2] [1 3] [1 4])
user> (for [:let [x 1]
            y [2 3 4]]
        [x y])
IllegalStateException Can't pop empty vector  clojure.lang.PersistentVector.pop (PersistentVector.java:380)



Screened by:

Comment by Andy Fingerhut [ 30/Dec/13 9:53 AM ]

Related (perhaps identical?) ticket CLJ-207 was declined.

Comment by Jay Fields [ 30/Dec/13 10:03 AM ]

It does look like a duplicate. I find it surprising that this doesn't work, but it does work for doseq:

main=> (doseq [:let [x 1] y [2 3 4]] (println x y))
1 2
1 3
1 4

I think you'll keep getting this bug report as long as that inconsistency exists.

Comment by Jay Fields [ 30/Dec/13 10:05 AM ]

for completeness, I think it's worth mentioning that I can't simply change the ordering (like Alex's example above), due to the cost of the value I'm calculating. I only want it to occur once, and I have to use a separate 'let (as Rich recommended)

Comment by Gary Fredericks [ 05/Jan/14 3:37 PM ]

Brandon Bloom pointed out that one difference between for and doseq is that for is lazy, and so for an initial :let it's not clear whether it should be evaluated immediately or after the first item is requested. doseq doesn't have that ambiguity.

Comment by Jay Fields [ 08/Jan/14 10:42 AM ]

@Gary, I think that's a good question, but either choice would be better than the current inconsistency. If you made it lazy, I can't really think of a downside. Even if it wasn't lazy, that would match the current performance characteristics of code that's already wrapping the for in a let.

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