Stream: ideas

Topic: type annotations without duplication

itmuckel (Mar 17 2023 at 22:25):

Hi! I've wondered since first time seeing haskell code:

fullName : Str, Str -> Str
fullName = \firstName, lastName ->
    "\(firstName) \(lastName)"

Why is it necessary for the function name to be repeated? Couldn't _ : Str, Str -> Str work as well? Or are there advantages to doing it like this?

itmuckel (Mar 17 2023 at 22:25):

Of course it wouldn't align the : with the = :tear:

Brendan Hansknecht (Mar 17 2023 at 22:31):

I think it is just a design and readability thing. Also, your alternative syntax is missing the variables for the arguments. It just has their type.

Brendan Hansknecht (Mar 17 2023 at 22:32):

But could be something like:
foo = firstName: Str, lastName: Str ->

Brendan Hansknecht (Mar 17 2023 at 22:32):

Though that is missing the return type also.

Brendan Hansknecht (Mar 17 2023 at 22:33):

Oh, you just meant unnamed on the line before.

Brendan Hansknecht (Mar 17 2023 at 22:34):

_ : Str, Str -> Str
fullName = \firstName, lastName ->
    ...

Brendan Hansknecht (Mar 17 2023 at 22:35):

That could parse too.

Brendan Hansknecht (Mar 17 2023 at 22:36):

I think it also falls under my first comment. Just design and readability and such. Alternatives definitely could be made.

Martin Stewart (Mar 17 2023 at 22:45):

Doesn't Roc let you just write fullName : Str, Str -> Strwhich it treats as a

fullName : Str, Str -> Str
fullName = \_, _ -> panic

?

If it does then using _ instead of the fullName wouldn't work.

Brendan Hansknecht (Mar 17 2023 at 22:53):

Sure sure, but i don't think the specific character there matters, more the general idea of not repeating the name twice.

Zeljko Nesic (Mar 18 2023 at 01:36):

if you ask me even this might be legit

fullName : Str, Str -> Str
= \firstname, lastname -> "\(firstname) \(lastname)"

Georges Boris (Mar 18 2023 at 01:41):

Str, Str -> Str
fullName = \firstname, lastname -> "\(firstname) \(lastname)"

gg

Anton (Mar 18 2023 at 08:51):

I think the alignment of the existing design is really good, it's nice to have the columns of the first type and first argument name close together.

fullName : Str, Str -> Str
fullName = \firstName, lastName ->

Although my preferred way to display it in the editor would be something like this:

            Str        Str      -> Str
fullName = \firstName, lastName ->

Last updated: Jun 16 2026 at 16:19 UTC