Leonardo Taglialegne (Mar 23 2023 at 23:33): Leonardo Taglialegne (Mar 23 2023 at 23:33):Reading the Roc-for-Elm tutorial. So far most things are reasonable and/or improvements over Elm.
Leonardo Taglialegne (Mar 23 2023 at 23:34):The first weird thing I found is the syntax for open records which is just... Weird
{ name : Str, email : Str }a -> { name : Str, email : Str }a especially because I'd feel like writing it as
{ name : Str, email : Str } a -> { name : Str, email : Str } a which feels... even weirder :thinking:
Leonardo Taglialegne (Mar 23 2023 at 23:35):Qualified-but-not-dotted record update is :chefs-kiss:
Leonardo Taglialegne (Mar 23 2023 at 23:36):...and the table function will fill in its default values for
xandy. There is no need to use adefaultConfigrecord.
Except it has default for title and description. Eh. Guess a PR is incoming :laughing:
Leonardo Taglialegne (Mar 23 2023 at 23:39):https://github.com/roc-lang/roc/pull/5184
Leonardo Taglialegne (Mar 23 2023 at 23:41):Optional record fields are just Comfortable
Leonardo Taglialegne (Mar 23 2023 at 23:42):Destructuring is the only way to implement a record with optional fields. (For example, if you write the expression
config.titleandtitleis an optional field, you'll get a compile error.)
I get it but also... it could just be a Maybe? But then you'd bless a specific type which is suboptimal I guess? :thinking:
Leonardo Taglialegne (Mar 23 2023 at 23:43):
when/isinstead ofcase/of
Fair enough
Leonardo Taglialegne (Mar 23 2023 at 23:43):pipe for multiple patterns
YES thank you
Leonardo Taglialegne (Mar 23 2023 at 23:43):pattern guards
Yes
Leonardo Taglialegne (Mar 23 2023 at 23:44):can use
ifguards to match against constants
YESSSSS
Leonardo Taglialegne (Mar 23 2023 at 23:44):So tempted to rewrite nuPlot in Roc now :laughter_tears:
Leonardo Taglialegne (Mar 23 2023 at 23:45):and Bool is an opaque type. The values Bool.true and Bool.false work like True and False do in Elm.
weeeeird. Is this to avoid when/is on booleans? Guess one would when/is/if then :thinking:
Leonardo Taglialegne (Mar 23 2023 at 23:46):Having to qualify booleans like Bool.true and Bool.false feels weeeeeird for such a basic thing. Is the idea to push towards custom types harder?
Leonardo Taglialegne (Mar 23 2023 at 23:47):
Tasktype args are flipped
...mmmmmmmmmmh. I get it. But also. Mhhhhhhh
Leonardo Taglialegne (Mar 23 2023 at 23:48):combined error tags
AMAZING
Leonardo Taglialegne (Mar 23 2023 at 23:49):open tag unions syntax
see my comment about re: open record syntax. weeeeeeeeeeeeeeeeeeeird
Leonardo Taglialegne (Mar 23 2023 at 23:49):You don't need to declare tag unions before using them. Instead, you can just write a tag (essentially a variant) anywhere you like, and Roc will infer the type of the union it goes in.
Thank you
Leonardo Taglialegne (Mar 23 2023 at 23:50):> y = Foo "hi" Bar
Foo "hi" 5 : [Foo Str [Bar]*]*
This is another typo but I'm not sure what the intent was
Leonardo Taglialegne (Mar 23 2023 at 23:52):Similarly to how if you put { name = "" } into elm repl, it will infer a type of { a | name : String } - that is, an open record with an unbound type variable and name : Str field
It doesn't tho. It only infers that open record if you hello {name} = "Hello " ++ name
Brendan Hansknecht (Mar 23 2023 at 23:54):For optional record fields:
I get it but also... it could just be a Maybe? But then you'd bless a specific type which is suboptimal I guess?
This is because optional record fields aren't really optional. They really should be called record fields with default values for a specific function. This is all known and dealt with at compile time. Depending on types, we generate specific functions with those values set as need. If you had a record field that was truly optional at runtime, you would use a Result or Maybe like value to hold the data.
Leonardo Taglialegne (Mar 23 2023 at 23:56):Makes sense. I learned from Elm that it's fine to restrict features to nudge users towards better code
Leonardo Taglialegne (Mar 23 2023 at 23:57):Opaque types
:shrug: ok
Brendan Hansknecht (Mar 23 2023 at 23:58):Also, for bool, the context is here: #ideas > opaque bools
Brendan Hansknecht (Mar 23 2023 at 23:58):Doc at the top here: https://roc.zulipchat.com/#narrow/stream/304641-ideas/topic/opaque.20bools/near/286971346
Leonardo Taglialegne (Mar 24 2023 at 00:05):Yupp, I'm a convert. Love it
Leonardo Taglialegne (Mar 24 2023 at 00:06):app vs interface modules
Mmmmh :thinking: wouldn't this prevent multiple entrypoints? Which is a nice thing sometimes
Leonardo Taglialegne (Mar 24 2023 at 00:07):@Brendan Hansknecht I appreciate your responses, thanks for taking the time :purple_heart:
Leonardo Taglialegne (Mar 24 2023 at 00:07):All imports and exports in Roc are enumerated explicitly; there is no .. syntax.
YES
Leonardo Taglialegne (Mar 24 2023 at 00:07):(Later on, we'll talk about how opaque types work in Roc.)
_technically_ you already did
Leonardo Taglialegne (Mar 24 2023 at 00:08):Elm does permit overriding open imports - e.g. if you have import Foo exposing (bar), or import Foo exposing (..), you can still define bar = ... in the module. Roc treats this as shadowing and does not allow it.
Good
Leonardo Taglialegne (Mar 24 2023 at 00:12):Roc has no << or >> operators, and there are no functions in the standard library for general-purpose pointfree function composition.
:clap:
Leonardo Taglialegne (Mar 24 2023 at 00:13):The only "good" use of << is List.filter (not << String.isEmpty)
Leonardo Taglialegne (Mar 24 2023 at 00:14):Which. Like. List.filter String.Extra.isNotEmpty
Leonardo Taglialegne (Mar 24 2023 at 00:15):Separately, you don't need parens or an operator to pass when or if as arguments. Here's another example:
Nize
Leonardo Taglialegne (Mar 24 2023 at 00:16):list =
num <- List.map numbers
num + 1
oooooooooh, this is NICE. At first I was like "oh, ok, monadish syntax" but no, this is way better
Leonardo Taglialegne (Mar 24 2023 at 00:17):incrementedNumbers =
num <-
[1, 2, 3]
|> List.reverse
|> List.map
num + 1
ooooh, it combines So Nicely
Leonardo Taglialegne (Mar 24 2023 at 00:18):Oh, _that's_ why you Task.await! It's just an andThen mean to be used in backpassing style!
Leonardo Taglialegne (Mar 24 2023 at 00:20):
Dec
:clap:
Leonardo Taglialegne (Mar 24 2023 at 00:22):If you encounter integer or Dec overflow in Roc, by default you get a runtime exception.
Oh. Oh no. Oh PLEASE no. Nope. Noooope. Like. I get it. But. Ugh
Leonardo Taglialegne (Mar 24 2023 at 00:27):No Char. This is by design.
Good
Leonardo Taglialegne (Mar 24 2023 at 00:30):
dbgauto-yeeted in prod
Good
Leonardo Taglialegne (Mar 24 2023 at 00:32):
crashkeyword
mmmmmmmh :thinking: I like that it's uncatchable, but I would also be wary of a library just randomly crashing on me?
Brendan Hansknecht (Mar 24 2023 at 00:42):Leonardo Taglialegne said:
If you encounter integer or Dec overflow in Roc, by default you get a runtime exception.
Oh. Oh no. Oh PLEASE no. Nope. Noooope. Like. I get it. But. Ugh
For reference, this was kinda a fitted compromise that matches the other numeric types. We tried enforcing more checked arithmetic on numeric types, but fundamentally it adds too much friction to the language. So Dec behaves the same as all the other regular number types. When you do a + b, whether it is a U64, I8, or Dec, it may overflow (which will run time panic). You, of course, can opt into checked arithmetic that will return a result.
Brendan Hansknecht (Mar 24 2023 at 00:47):Leonardo Taglialegne said:
crashkeywordmmmmmmmh :thinking: I like that it's uncatchable, but I would also be wary of a library just randomly crashing on me?
There was a lot of debate around this and it may change at some point, but fundamentally crash can be useful in libraries. For example, Dict is written in Roc. It uses crash for states that should be unreachable. Without crash, there is not really a clear thing to do if somehow you reached a state that should be unreachable and would definitely otherwise be a bug. The general thought is that it will hopefully turn out like panic in rust libraries. Most rust libraries will never panic, or if they do it is an unrecoverable library bug that should be filed upstream.
Luke Boswell (Mar 24 2023 at 01:00):Interesting, for some reason I thought crash only worked in app modules.
Luke Boswell (Mar 24 2023 at 01:10):Here is Richard's proposal and zulip thread discussion for anyone who is interested in reading more about crash design.
Brendan Hansknecht (Mar 24 2023 at 01:21):Interesting, for some reason I thought
crashonly worked inappmodules.
Just double checked. I was able to put a crash directly in an interface in a platform.
Luke Boswell (Mar 24 2023 at 01:22):Yeah, I believe you. It is definitely in Dict. I guess I assumed somewhere along the way. Good to know. I think it's a good trade-off.
Leonardo Taglialegne (Mar 24 2023 at 01:26):Anyway. Considering that you're only a novice once I wanted to share my impressions :)
I will now probably try and play with roc a bit
Btw, thanks to all the people who worked on flake.nix, I was able to install roc by just adding it to home-manager like this: https://github.com/miniBill/dotfiles/commit/c0f1775048011f9e0580d842604870109be12a3f
Notification Bot (Mar 24 2023 at 22:02):7 messages were moved from this topic to #ideas > open record syntax by Richard Feldman.
Leonardo Taglialegne (Mar 25 2023 at 12:23):The List module does not expose `length`:
I'd write "The List module does not expose a value or function named length".
The current verbiage sounds like "the function exist but is not exposed".
Leonardo Taglialegne (Mar 25 2023 at 12:24):» -(List.len ['a'])
18446744073709551615 : Nat
... I know why it happens but it's still surprising
Leonardo Taglialegne (Mar 25 2023 at 12:24):» -(List.len ['a']) + 3.0
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Erroneous', crates/compiler/mono/src/ir.rs:10433:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Whoops :D
Richard Feldman (Mar 25 2023 at 13:15):Leonardo Taglialegne said:
» -(List.len ['a']) 18446744073709551615 : Nat... I know why it happens but it's still surprising
oh this is actually a bug! I've opened https://github.com/roc-lang/roc/issues/5197 for it
Richard Feldman (Mar 25 2023 at 13:17):Leonardo Taglialegne said:
» -(List.len ['a']) + 3.0 thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Erroneous', crates/compiler/mono/src/ir.rs:10433:10 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
This is also a bug! Would you mind opening an issue for it?
Leonardo Taglialegne (Mar 25 2023 at 14:04):Richard Feldman said:
oh this is actually a bug! I've opened https://github.com/roc-lang/roc/issues/5197 for it
Would it be possible to instead forbid using negate on Nat?
Leonardo Taglialegne (Mar 25 2023 at 14:06):Richard Feldman said:
This is also a bug! Would you mind opening an issue for it?
Minimized and posted https://github.com/roc-lang/roc/issues/5199
Richard Feldman (Mar 25 2023 at 14:17):Leonardo Taglialegne said:
Richard Feldman said:
oh this is actually a bug! I've opened https://github.com/roc-lang/roc/issues/5197 for it
Would it be possible to instead forbid using
negateonNat?
I considered it a long time ago, but I concluded it wouldn't be worth it because it would introduce an entire type-level distinction between signed and unsigned numbers basically just to prevent calling a function unnecessarily :big_smile:
Richard Feldman (Mar 25 2023 at 14:18):(also there would be a nonzero cost to compile times to introducing that distinction)
Richard Feldman (Mar 25 2023 at 14:19):I could see an argument for having negation of an unsigned integer always crash, even if you give it 0
Richard Feldman (Mar 25 2023 at 14:19):and just say "you tried to negate an unsigned number, and unsigned numbers can't be negative"
Richard Feldman (Mar 25 2023 at 14:20):that would mean that if you ever called Num.neg on an unsigned number, it wouldn't give you a compile-time error, but in practice you'd always find out about it right away if you ever tried it
Richard Feldman (Mar 25 2023 at 14:21):you wouldn't even have the "whoops it was 0 in my test cases, so I didn't notice" edge case
Richard Feldman (Mar 25 2023 at 14:22):actually that seems worth trying and seeing if it causes problems; I've updated the issue
Ayaz Hafiz (Mar 25 2023 at 18:10):Richard Feldman said:
I considered it a long time ago, but I concluded it wouldn't be worth it because it would introduce an entire type-level distinction between signed and unsigned numbers basically just to prevent calling a function unnecessarily :big_smile:
For builtin operations, Roc would not need to introduce a type-level distinction - the compiler already has support we added for checking, for example, does this integer fit in a U8? So, we could apply the same analyses for catching negations of unsigned integers, without having to change the type-level syntax or integer types a Roc programmer needs to know.
The same kind of thing could be used to catch checks like myU8 < 0u8 which will never succeed.
Richard Feldman (Mar 25 2023 at 18:40):hm, what about something like this?
negatesArgInSomeCodePath : Num a -> Num a
x = negatesArgInSomeCodePath someSignedInt
y = negatesArgInSomeCodePath someUnsignedInt
in that case, would it give an error for y? if so, what would the error be? :thinking:
Ayaz Hafiz (Mar 25 2023 at 18:45):yeah, that might not detectable at compile time - but something like
f : U8 -> U8
f = \x ->
...
g = -x * -x
...
g
the compiler could provide today with no language changes
Richard Feldman (Mar 25 2023 at 18:47):ahh sure, that's a good point!
Richard Feldman (Mar 25 2023 at 18:47):so just like "if we know for sure it's gonna be a problem, we can tell you; otherwise, you might get a runtime error"
Last updated: Jul 26 2025 at 12:14 UTC