Aurélien Geron (Aug 03 2026 at 03:40): Aurélien Geron (Aug 03 2026 at 03:40):I'm trying to migrate the RecordBuilder example but the <- operator no longer exists so I'm not sure what to do. Should I drop this example for now (e.g., if there's a plan to add the <- operator in the future) or should I just implement a desuraged version?
Luke Boswell (Aug 03 2026 at 04:16):Record builders look different now
Luke Boswell (Aug 03 2026 at 04:17):Here is an example
character_generator : Str -> Random.Generator(Character)
character_generator = |name| {
d20 = Random.bounded_u8(1, 20)
lucky_generator = Random.map(Random.bounded_u8(1, 20), |roll| roll == 20)
{
name: Random.static(name),
strength: d20,
agility: d20,
hit_points: Random.bounded_u8(8, 16),
lucky: lucky_generator,
}.Random
}
Any nominal type with a map2 can participate automatically
Richard Feldman (Aug 03 2026 at 04:18):@Aurélien Geron we just changed the syntax! See https://github.com/lukewilliamboswell/weaver for an example - you basically make a nominal type with a map2 method
Richard Feldman (Aug 03 2026 at 04:18):ha, Luke beat me to it :laughing:
Luke Boswell (Aug 03 2026 at 04:18):Richards example is better though :shrug:
Aurélien Geron (Aug 03 2026 at 04:19):Cool, thanks to both of you! :+1:
Aurélien Geron (Aug 03 2026 at 23:02):It took me a while, but I finally got a working minimal example. I'll post it here in case anyone else needs a simple example:
EnvReader(a) : Dict(Str, Str) -> Try(a, [MissingEnv(Str)])
Reader :: {}.{
map2 : EnvReader(a), EnvReader(b), (a, b -> c) -> EnvReader(c)
map2 = |reader_a, reader_b, combine|
|env| {
a = reader_a(env)?
b = reader_b(env)?
Ok(combine(a, b))
}
}
read_str : Str -> EnvReader(Str)
read_str = |key| {
|env| {
env.get(key).map_err(|_| MissingEnv(key))
}
}
main! = |_| {
app_config = {
host: read_str("HOST"),
port: read_str("PORT"),
}.Reader
env = Dict.from_list([
("HOST", "127.0.0.1"),
("PORT", "8000"),
("FOO", "123"),
("BAR", "456"),
])
dbg app_config(env)
Ok({})
}
Last updated: Aug 12 2026 at 12:35 UTC