id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	difficulty
1422	for-each behaves inconsistently when compiled versus interpreted on lists of unequal length	sjamaan		"There's a compiler syntax registered for {{{for-each}}} (and for {{{map}}}, too) which rewrite/expands most {{{for-each}}} calls to a loop for performance reasons.

While working on the ""scheme"" module at some point I forgot to patch this to apply to {{{scheme#for-each}}} so it didn't get applied, and I got an error in both the scrutinizer and a test. In both cases, for-each was being used on two lists of unequal length, causing for-each to bail out with an error.

Example:

{{{
(for-each (lambda (x y) (print (+ x y))) '(1 2 3) '(1 2))
}}}

When compiled, this prints {{{2\n4}}}, when interpreted this gives the following (quite useless) error:

{{{
2
4

Error: (for-each) lists are not of same length: (())
}}}

I think this error message really needs to be improved (which will be a bit more costly in terms of garbage collection because the head is held onto regardless of how far we got in the loop). I believe {{{map}}} has the same issue.

Also, giving the error after executing the procedure is questionable; maybe it should first check the lengths of the lists and ''then'' start traversing them?

Finally, we need to decide how we want {{{for-each}}} to behave: like the compiler syntax or like the procedure. Either is fine by me, and judging what the standard says, I think both are fine too. It doesn't explicitly say ""it is an error"" when the lists aren't the same length, but it just says ""the lists must be the same length"".  R7RS explicitly allows the lists to be of unequal length (presumably taken from SRFI-1's extended definition), so maybe we should just allow this as an allowed extension to R5RS and get rid of the check."	defect	closed	major	5.2	core libraries	5.0.0	fixed	compiler syntax, for-each, inconsistency		medium