Warning/error on seq with no body

[sarna]

It'd be nice if Janet warned you (threw an error, printed a warning, ...) when you don't provide the body for seq, for example:

(seq [i :range [0 5]])

Right now it will happily return @[nil nil nil nil nil]. I can't think of a reason why a user wanted to write it this way.

The same thing happens with loop, not sure about other looping constructs.

[sogaiu]

I guess if one wanted 5 nils, it would be clearer to express it like:

(seq [i :range [0 5]] nil)

[sogaiu]

@sarna I think this commit may have a relevant change:

$ janet
Janet 1.35.2-342a29c7 linux/x64/gcc - '(doc)' for help
repl:1:> (seq [i :range [0 5]])
repl:1:1: compile warning (normal): empty loop body
@[nil nil nil nil nil]

[sogaiu]

Although the change introduced makes sense for seq, I'm not sure it does for loop.

I found an example here where an empty-body loop is being used for side-effects:

    (loop [line :iterate (file/read infile :line)
           :until (string/has-prefix? ">THREE " line)])

[sarna]

With loop I think no warning is okay. seq is explicitly a list comprehension, so I'd expect the result to be used - not really true with loop.

[sogaiu]

FWIW, in the above case, the warning (that shows up on stderr) that appears for an empty-body loop broke a "driver" program that was parsing stderr. It had expectations that only certain output would show up on stderr (i.e. that coming from progams being executed).

I think in this case, the warning should actually not happen -- there seem to be legitimate reasons to have empty-bodied loops (in contrast to seq).