mutable variables are not considered local, thus cannot be evaluated in conditionals

[nohehf]

The following tsg stanza, while executed on an empty python file:

(module) {
    var a = #true
    if a {
        print "a is true"
    }
}

Will error:

Expected local value at (6, 8)
test.tsg:6:8:
6 |     if a {
  |        ^

Where it would work just fine with an immutable "let":

(module) {
    let a = #true
    if a {
        print "a is true"
    }
}

Will print "a is true" as expected.

This is a weird behavior, as a should have the same scope in both cases. It also prevents to do patterns such as:

var a = ; some expression that might be null
if (is-null a) {
    set a = ; some default value
}

Additionally I think that it is quite limiting not being able to execute conditionals on non-local values, why is that ?

[hendrikvanantwerpen]

The analysis could indeed be more precise around mutable variables. It might require changing the structure of the checker a little. I think it's currently a single-pass thing, and that wouldn't work anymore. The checks would have to be moved to the end of a block.

Additionally I think that it is quite limiting not being able to execute conditionals on non-local values, why is that ?

Two for keeping control local to a stanza reasons here:

• It makes reasoning about stanza behavior much easier. The shape of the graph that is created by a stanza is completely determined by the nodes that were matched.

• It keeps the implementation simpler. Stanza's can be reduced to graph statements and expression thunks that can be lazily evaluation. That ensures that stanza evaluation order doesn't matter. If we wanted to allow conditionals on non-local values, we'd have to push allt he control logic into the lazy expressions. It's possible, but the implementation becomes quite complex.