Stream: beginners

Topic: ✔ Struggling with type inference for expect / tests

Fábio Beirão (Jun 07 2023 at 17:55):

I'm a bit struggling with creating an interface for a "generic Stack" (regardless of whether it already exists in a package or not, I'm trying to create my own for educational purposes)

If I have the two expect at the bottom uncommented at the same time, roc check stops being my friend.
And if I have one of the expect uncommented, and I add the type signature to empty (I think the type would be empty : Stack *, then roc check also stops being my friend.

interface Stack
    exposes [
        Stack,
        empty,
        withCapacity,
        isEmpty,
        top,
        put,
        pop,
    ]
    imports []

Stack a := List a

# 👇 If I uncoment this type annotation (together with one of the tests below), then roc check is no longer happy

#empty : Stack *
empty = fromList []

fromList : List a -> Stack a
fromList = \list -> @Stack list

withCapacity : Nat -> Stack a
withCapacity = \elements ->
    @Stack (List.withCapacity elements)

isEmpty : Stack * -> Bool
isEmpty = \@Stack stack ->
    stack |> List.isEmpty

top : Stack a -> Result a [StackWasEmpty]
top = \@Stack stack ->
    stack |> List.last |> Result.mapErr (\_ -> StackWasEmpty)

put : Stack a, a -> Stack a
put = \@Stack stack, elem ->
    @Stack (stack |> List.append elem)

pop : Stack a -> Result { elem : a, stack : Stack a } [StackWasEmpty]
pop = \@Stack stack ->
    when (stack |> List.last) is
        Ok elem -> Ok { elem, stack: @Stack (stack |> List.dropLast) }
        Err ListWasEmpty -> Err StackWasEmpty

expect (empty |> isEmpty) == Bool.true

## 👇 I can't have these two uncommented at the same time 😖

expect (empty |> put 1 |> put 2 |> pop |> Result.map .elem) == Ok 2
#expect (empty |> put "A" |> put "B" |> pop |> Result.map .elem) == Ok "B"

This is the nightly version of roc. Am I doing something impossible, or is this a well-known issue?

Brendan Hansknecht (Jun 07 2023 at 18:04):

Empty needs to be a function

Brendan Hansknecht (Jun 07 2023 at 18:04):

Then it would be used as empty {}

Brendan Hansknecht (Jun 07 2023 at 18:04):

To create a new empty stack

Fábio Beirão (Jun 07 2023 at 18:04):

Yeah, I'm looking at Dict, and indeed, Dict.empty is also used as Dict.empty {}

Fábio Beirão (Jun 07 2023 at 18:05):

Ok, let me make some adjustments :D :sweat_smile:

Fábio Beirão (Jun 07 2023 at 18:07):

:tada: As a wise man used to say, "There's nothing like speaking with those who know". Thanks @Brendan Hansknecht that was the solution :pray::pray:

Notification Bot (Jun 07 2023 at 18:07):

Fábio Beirão has marked this topic as resolved.

Brendan Hansknecht (Jun 07 2023 at 18:09):

@Ayaz Hafiz do you think there is a way for the compiler to give a better error message related to this and #ideas > Let-generalization - let's not? ? Should we file an issue for clarity in the error message here or would that not really be possible?

Ayaz Hafiz (Jun 07 2023 at 18:16):

yeah, we should definitely give better error messages. There's a section in the "Let's not?" doc that describes how we can do it, so that can be referenced in the issue

Brendan Hansknecht (Jun 07 2023 at 18:21):

Can you write up the issue since you have more context?

Ayaz Hafiz (Jun 07 2023 at 18:28):

sure, will do

Last updated: Jul 26 2025 at 12:14 UTC