Stream: helpdesk (published)

Topic: ✔ `ifelse` with functions

Chad Scherrer (Apr 13 2022 at 16:11):

I think I'm missing something obvious. This works fine:

julia> function f(a,b)
       a || return ifelse(b, "ft", "ff")
       b || return "tf"
       return "tt"
       end
f

julia> function check(fun)
       for a in (true, false)
           for b in (true, false)
               println((a,b) => fun(a,b))
           end
       end
       end
check

julia> check(f)
(true, true) => "tt"
(true, false) => "tf"
(false, true) => "ft"
(false, false) => "ff"

But I'd expect this to also work:

julia> function g(a,b)
       ifelse(a, () -> nothing, () -> return ifelse(b, "ft", "ff"))()
       ifelse(b, () -> nothing, () -> return "tf")()
       return "tt"
       end
g

julia> check(g)
(true, true) => "tt"
(true, false) => "tt"
(false, true) => "tt"
(false, false) => "tt"

The idea for g is to have a way to make ifelse lazy, so it could work with StaticBools as well as Bools. What am I missing?

Chad Scherrer (Apr 13 2022 at 16:19):

As a side note, I'm moving away from using StaticBool as an optimization and toward only using this when I need it for dispatch. But I do need it temporarily to for short-term backward compatibility

Richard Reeve (Apr 13 2022 at 16:21):

Surely the return x statements are inside the lambda functions you have created, so the first two lines don't return from the main g() function?

Chad Scherrer (Apr 13 2022 at 16:23):

...Are they not? It looks to me like they're inside the lambdas. Anyway, I'm having the opposite problem - g always returns tt, which must mean both ifelse lines reduce to nothing

Chad Scherrer (Apr 13 2022 at 16:29):

If a is false, I'd think

ifelse(a, () -> nothing, () -> return ifelse(b, "ft", "ff")) == () -> return ifelse(b, "ft", "ff")

So applying this (the () at the end) should....

Oh wait, I think I get it, it's interpreting the return as part of the anonymous function, instead of as part of g

Chad Scherrer (Apr 13 2022 at 16:30):

That threw me, since I never use return to just return a value from an anonymous function

Notification Bot (Apr 13 2022 at 16:31):

Chad Scherrer has marked this topic as resolved.

Mason Protter (Apr 13 2022 at 17:02):

The secret is to just never use return. It’s just a less useful goto.

Chad Scherrer (Apr 13 2022 at 17:15):

That's worked well for me in some cases, but here I want to short-circuit most of the code. Would you use a @goto for this?

Mason Protter (Apr 13 2022 at 17:24):

I’d just not use ifelse at all and instead write if x == true and then make it so that True() == true

Chad Scherrer (Apr 13 2022 at 17:25):

Nice, I hadn't thought of that

Chad Scherrer (Apr 13 2022 at 20:34):

julia> Static.True() == true
true

:thumbs_up:

Mason Protter (Apr 13 2022 at 20:43):

Another option would be something like

istrue(::True) = true
istrue(::False) = false
istrue(b::Bool) = b

if istrue(a)
    # ...
end

if you prefer how that looks

Mason Protter (Apr 13 2022 at 20:44):

You can also make anything you feel like 'truthy' in this way. E.g.

istrue(x::Integer) = x % Bool
istrue( ::Missing) = false

Last updated: Oct 02 2023 at 04:34 UTC