[racket-dev] sequence syntax for (mlist #t #f …) ?

From: John Clements (clements at brinckerhoff.org)
Date: Thu Jun 21 02:33:34 EDT 2012

Reality check before I do something dumb and re-invent the wheel:

I often want to write a for loop where the first element is treated specially. In such cases, it would be nice to have a sequence that had a #t and then an infinite number of #f's, so I could write

(for ([s my-sequence] [first? <true-then-always-falses>]) …)

Hey, no problem! Mutable lists can be used as sequences. But--and here I feel as though my brains have suddenly vanished--I can't think of the nice short way to write

(mlist #t #f …)

I'd be sort of okay with writing

(mcons #t #0=(mcons #f #0#))

… but that's not allowed, either. 

Yes, of course I can do it the ugly way:

(define p1 (mcons #f 'bogus))
(set-mcdr! p1 p1)
(define true-then-falses (mcons #t p1))

Of course, it would be unbelievably easy to write an mlist syntactic form, but there *must* be a form that's been around for 150 years that already does this. 

Sorry to have forgotten Scheme,


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