Support walrus operator creating new variables + recursive lambda functions

[sean-niemann]

Support walrus operator creating new variables + recursive lambda functions

[s-hadinger]

Lots of questions here.

First I tried to have an option where the walrus operator creates the variable on the fly, but it's not possible due to the nature of the single pass compiler of Berry. I can elaborate why, but in short, new variable declarations need to be before any statement, not in the middle.

Actually print(n:=2) does work in the REPL by accident. But
def f() print(n:=2) end
does print what I find is explicit enough
syntax_error: stdin:1: strict: no global 'n', did you mean 'var n'?

For recursive lambda, the fact that any variable must be known by the compiler before being used is in fact a good feature that saved be a ton of hidden bugs. But the drawback is that it requires to define it with a dummy value before. It is maybe possible to patch the compiler in this very specific case, but it will not patch the case of two recursive functions calling one another. This is again due to the single pass compiler that does not have a linker phase to check defined or not defined variables.

Last trick, if the function is global, you can use the global module to avoid the compilation error. But that's an edge case and wouldn't work inside a function (and performance wide it's not great because it adds an indirection):

import global
fn=/n->n?1:global.fn(n+1)