thoughts on extending comma syntax

[tiye]

Short version

Currently , syntax is only useful when it's used at head of an expression. If it's put inside an expression, it will be regarded just as a symbol of ,. Now I suppose, , can also be used inside expressions, as an operator of "nest former items".

Say we have code:

a b , c d

now it's parsed to:

["a" "b" "," "c" "d"]

in the context of extended comma syntax, it can be parsed to:

[["a" "b"] "c" "d"]

And then combined with $, we can have code:

a b , $ c d

which can be parsed to:

[["a" "b"] ["c" "d"]]

I'm not ready to add it yet. Maybe it could be a new operator just like ,$ since they are used in pair in the cases above. I'm not sure.

Details

The idea bumped when I was trying to describe https://en.wikipedia.org/wiki/B,_C,_K,_W_system in Cirru. For SKI, we have

S f g x = f x (g x);
K x y = x;
I x = x;

written in Cirru, with a new lambda syntax:

def S $ \f \g \x
  (f x) (g x)

def K $ \x \y x

def I $ \x x

And for BCKW, we have:

B x y z = x (y z);
C x y z = x z y;
K x y = x;
W x y = x y y;

written in Cirru:

def B $ \x \y \z
  x $ y z

def C $ \x \y \z
  (x z) y

def K $ \x \y x

def W $ \x \y
  (x y) y

And if we extend comma syntax, we could make it simpler:

; SKI

def S $ \f \g \x
  f x , $ g x

def K $ \x \y x

def I $ \x x

; BCKW

def B $ \x \y \z
  x $ y z

def C $ \x \y \z
  x z , y

def K $ \x \y x

def W $ \x \y
  x y , y

it's simpler, right?

And this extended comma may also help in formatting continuous nested expressions like:

a
    b c
    d e
  (e d)
    f g
  (h i)
    j k

which currently parsed to:

[["a" [["b" "c"] ["d" "e"]] [["e" "d" "," ["f" "g"]]] [["h" "i"] ["j" "k"]]]]

With extended commas, it can be written as:

a
    b c
    d e
  , $
    e d , $ f g
  , $
    h i , $ j k

quite inspiring for me.

Considerations

It seems this extension will not break current semantics of Cirru, and will unlikely to break existing codebase. Only case I think that will break is there was a [,] macro in calcit-runner that relies on commas, but it won't be big problem to change that macro for a better syntax like ,$.

Not ready to make this change yet. And current solution is still handing in most cases. To be decided in the future.