Stream: beginners

Topic: Are these bugs?

Ben Plotke (Mar 06 2024 at 16:58):

Hi, I am just beginning to experiment with the language, and I have encountered several surprising behaviors. I didn't see these on the issues page, but I might have just missed them.

What I was trying to do was implement a function which takes two lists, and determines if the first is in the second. It's the same as Haskell's isInfixOf.

This was my first attempt

app "sublist"
    packages { pf: "https://github.com/roc-lang/basic-cli/releases/download/0.8.1/x8URkvfyi9I0QhmVG98roKBUs_AZRkLFwFJVJ3942YA.tar.br" }
    imports [pf.Stdout]
    provides [main] to pf

main = Stdout.line "hi"

contains = \a, b ->
    List.len a <= List.len b && (List.startsWith b a || contains a (List.dropFirst b 1))

expect contains [1] [1]

That will stackoverflow, even though List.startsWith [1] [1] is true, which should short circuit ||, which I assume is an inclusive or.

Next I tried

contains = \a, b ->
    when (a, b) is
        ([], _) -> Bool.true
        (_, []) -> Bool.false
        (_, [_, .. as tail]) ->
            List.len a <= List.len b && (List.startsWith b a || contains a tail)

For this it complained that my pattern matching isn't exhaustive because "Other possibilities include ( [_, ..], _ )". I do not understand how that other possibility is not part of my patterns.

The below works, but I am curious to know if I have misunderstandings or if the above are bugs. Thanks!

contains = \a, b ->
    when (a, b) is
        ([], _) -> Bool.true
        (_, []) -> Bool.false
        (_, _) ->
            List.len a <= List.len b && (List.startsWith b a || contains a (List.dropFirst b 1))

Richard Feldman (Mar 06 2024 at 18:37):

|| doesn't currently short-circuit. It's a change we've talked about but haven't implemented yet!

Anton (Mar 06 2024 at 18:44):

For this it complained that my pattern matching isn't exhaustive because "Other possibilities include ( [_, ..], _ )"

I'll file an issue for that

Anton (Mar 06 2024 at 19:00):

#6561

Ben Plotke (Mar 06 2024 at 21:44):

Richard Feldman said:

|| doesn't currently short-circuit. It's a change we've talked about but haven't implemented yet!

Oh, well that would explain that. Which gives me the idea to do it like this. (I also found the format command)

contains = \a, b ->
    List.len a
    <= List.len b
    && (
        if
            List.startsWith b a
        then
            Bool.true
        else
            contains a (List.dropFirst a 1))

Thanks for responding. Cool language!

Last updated: Jul 26 2025 at 12:14 UTC