.subst() does not descend into parameter lists

[ngn]

Example:

macro m (f) -> f.subst x: macro.codeToNode -> y
m (x) -> x + 1

compiles to

(function(x) {
  return y + 1;
});

I was expecting

(function(y) {
  return y + 1;
});

[vanviegen]

Hmm, that does look somewhat unexpected. The reason it works this way, is that the x in function(x) is not an expression. So it would create a pretty broken AST to put an arbitrary expression in its place.

I can't think of any good reasons why you'd want to do something like this though. What is it that you're trying to accomplish?

[ngn]

I wanted to use ⍺ (U+237A) and ⍵ (U+2375) as identifiers in CoffeeScript, even though they are not valid JS identifiers (these are APL symbols, not Greek letters). My first attempt was:

macro withAlphaAndOmega (f) ->
  f.body.subst
    '⍺': macro.codeToNode -> alpha
    '⍵': macro.codeToNode -> omega

withAlphaAndOmega -> # enclosing a section with this macro makes it possible to use ⍺ and ⍵ inside
  # ...
  someFunction = (⍺, ⍵) -> ⍺ + ⍵
  # ...

Anyway, I managed to do it with an ugly hack.

However, there are other valid reasons for .subst()-ing parameter names. E.g. if you want hygienic macros (and I mean truly hygienic):

macro multiple (f) ->
  (macro.codeToNode ->
    (n) -> for [0...n] then fBody
  ).subst fBody: f.body

greet = -> console.info 'hi'
greet() # prints 'hi'
multiGreet = multiple -> console.info 'hi'
multiGreet 3 # prints 'hi' three times

n = 'nick'
greetByName = -> console.info "hi, #{n}"
greetByName() # prints 'hi, nick'
multiGreetByName = multiple -> console.info "hi, #{n}"
multiGreetByName 3 # prints 'hi, 3' three times, oops

This could be fixed by .subst()-ing every local variable that occurs in the macro with some tmp variable:

macro multiple (f) ->
  (macro.codeToNode ->
    (n) -> for [0...n] then fBody
  ).subst fBody: f.body, n: macro.csToNode "tmp#{macro.tmpCounter++}"

Unfortunately, not descending into parameter lists makes the latter impossible.

[vanviegen]

Hmm, your second example is pretty nasty indeed. Perhaps substitution of local variable variables with something guaranteed to be distinct should be a feature of codeToNode? Manually replacing variable names by some tmp name doesn't feel like something I want to be worrying about every time. But then again, what if we want to use the same variable, how would we do that when it's auto-renamed out of the way..... Hmm.. :-S

How would you have subst work on parameters? Just throw when the replacement is not an identifier and it is to be used as a parameter? I'm still not really sure we need this. The thing about a local variable is that it's local, so what should we care how it's called in the generated code, right? :-)

[vanviegen]

As you can see in the patch above, I reconsidered. The previous behavior was probably more inconsistent then the new behavior is lacking. :-)

[ngn]

Excellent! Thank you. I'll think about what could be done to ensure macro hygiene, maybe Scheme can give some inspiration. Practice with blackcoffee will show what's most convenient.