Stream: beginners

Topic: Constants in Interface Module

Luke Boswell (Dec 11 2022 at 04:57):

Are there any runtime costs for creating constants in a separate interface module like below? I guess the alternative would be to hardcode these where they are needed, but I feel this might be clearer and more maintainable.

interface Unicode
    exposes [
        Character,
        utf8,
    ]
    imports []

UnicodeCharacter : {
    LF,
}

utf8 : UnicodeCharacter -> List U8
utf8 = \character ->
    when character is
        LF -> Str.toUtf8 "\u(000A)"
        CR -> Str.toUtf8 "\u(000A)"

Luke Boswell (Dec 11 2022 at 06:42):

I feel like Roc could support space : List U8 = Str.toUtf8 "\u(0020)" to define constants. Is this a bad idea?

Brendan Hansknecht (Dec 11 2022 at 07:59):

Cost for the interface, no. Roc looks at the program holistically

Brendan Hansknecht (Dec 11 2022 at 08:01):

As for your constant. I don't think it is a constant in current roc. I think roc will compile it into a top level thunk. So it will actually get called and be run. Sadly we don't have a comp time solutions here currently

Brendan Hansknecht (Dec 11 2022 at 08:01):

Is it a bad idea, depends on your use case and how often that would get called. I don't know if we currently memoize top level thunks

Brendan Hansknecht (Dec 11 2022 at 08:06):

There is a small change llvm will optimize it away, but probably not due to memory writes

Luke Boswell (Dec 11 2022 at 08:28):

Is there a way to make a constant in Roc? It would be sweet if we could use these in pattern matching, e.g.

EMPSACE : List U8 = [226, 128, 131]

when input is
    [EMSPACE, ..] -> do somthing... # U+2003

Luke Boswell (Dec 11 2022 at 08:35):

I was doing the below which works well enough. Are there any runtime issues here?

emSpace = [226u8, 128u8, 131u8]
input = [226, 128, 131, 10, 1,2,3,4]

if List.startsWith input emSpace then
    YES
else if List.startsWith input [10] then
    OTHERS # just chain a lot of these together...
else
    NOPE

Brendan Hansknecht (Dec 11 2022 at 08:44):

Those will be proper constants that are directly ingested into the binary.

Brendan Hansknecht (Dec 11 2022 at 08:44):

Well, emSpace and input will be.

Brendan Hansknecht (Dec 11 2022 at 08:45):

I don't think the if and other functions would reduce down to constants, but actually not sure. It would totally depend on llvm optimizations, and my gut feeling is that llvm + lists = not optimized away

Brendan Hansknecht (Dec 11 2022 at 08:46):

So i don't think it would compile the if to just always return YES even though in theory that is just a constant expression.

Luke Boswell (Dec 11 2022 at 08:56):

In this example the input would be variable and unknown, but it's good to know that emSpace will be a constant. :+1:

Richard Feldman (Dec 11 2022 at 09:29):

eventually I'd like to do compile-time evaluation to make it so that all top-level declarations are either functions or statically allocated constants, but that's a ways off :big_smile:

Last updated: Jul 26 2025 at 12:14 UTC