[plt-scheme] Possible bug in for/fold

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Fri Jun 27 12:12:23 EDT 2008

At Fri, 27 Jun 2008 16:20:02 +0100, "Noel Welsh" wrote:
> If for/fold has no accumulators but it does have for-clauses that
> iterate one or more times it gets upset:
>  -> (for/fold () ([i (in-range 4)]) (vector-set! (vector 0) 0 1))
> context (lexical binding) expected 0 values, received 1 value: #<void>
> This is because the body expression (in this case the vector-set!)
> returns a value which for/fold is not expecting.  This is kinda
> contrived, but has arisen in the context of a macro I'm writing that
> expands into for/fold (not for/fold/derived yet -- one step at a
> time!)
> It could well be argued this is not a bug, but I suggest one of two
> improvements:
> 1. for/fold accepts the above case and returns vold
> 2. for/fold rejects this case and gives an error at macro expansion time

It's ok to use `for/fold' with zero accumulators as long as the body
produces zero results:

 (for/fold () ([i (in-range 4)]) 
   (vector-set! (vector 0) 0 1)

That seems like the right thing to me. Is there some reason that your
macro can't/shouldn't match the zero accumulators with zero results?


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