internetimagery / do-not Goto Github PK

do(
    v1 + v2
    for v1 in Just(10)
    if (v2 := v1 + 10) # This assignment works in regular list expressions but if it resides within the "guard" function, which only returns a boolean. How shold this value be propagated?
)

Currently this will not work as the variable is created within the scope of the filter function. Which itself only returns a boolean.

Certainly it could be rearranged.

do(
    v1 + v2
    for v1 in Just(10)
    for v2 in Just(v1 + 10)
    if v2
)

But I"m sure there are some more useful examples than this toy one, that are only expressible in this form.

One small edge case remains unsolved.

v2 = 123
do(
    v1+v2
    for v1 in Just(123)
    for v2 in Just(v2)
)

When an outside variable is shadowed, treated as a nonlocal. It is not included in the closure as is standard. So when this is converted into a function, this variable is long out of scope. Thankfully the error message is helpful so it's not the end of the world (and this is poor coding design). But it would be good to have this fixed if a way can be found.

One inconsistency is that the first expression will always be fine, since it's evaluated before any adjustments are made.

v1 = 123
do(
    v1
    for v1 in Just(v1) # Fine, as it's in the first expression...
)

Note that this also works as the variable is non-shadowed:

v3 = 123
do(
    v1+v2
    for v1 in Just(123)
    for v2 in Just(v3)
)