ivory ember
I'm trying to pattern match using tuples as:
let a = #(1,2)
let b = #(2,1)
case a {
#(b.1, b.0) -> {}
_ -> {}
But the compiler complains:
I'm expecting a pattern here
Hint: A pattern can be a constructor name, a literal value
or a variable to bind a value to, etc.
Which means that I need to unpack b first. I wonder what's the reason I cannot use the tuple directly
oblique tulip
Looks fine to me
Can we see the whole error please
oh
sorry
Just saw
b.1 isn't a pattern, no
It's a value
ivory ember
but if I let m = #(b.1, b.0) then I can match agains m
I'd think that
let m = #(b.1, b.0)
case a {
m -> {}
}
should be the same as
case a {
#(b.1, b.0) -> {}
}
first snippet works, second does not
vernal burrow
That's not doing what you think it's doing. It's accepting any value and assigning to m
You'd have to do something if something == m -> to do it like that
hollow torrent
Matches in let are on the left side of the equals sign
So you’d see that let #(b.0, b.1) = b would also be a compiler error the same way matching on this with a case branch would be
ivory ember
that's because tuples are immutable ?
oblique tulip
I'd think that
```
let m = #(b.1, b.0)
case a {
m -> {}
}
```
should be the ...
It is not by design
viral ivy
The behaviour you’re expecting is called “pinning” where you can use the value or some other variable inside a pattern
oblique tulip
It is intentional
viral ivy
In gleam we dont have that, instead you would use guards
ivory ember
OIC
oblique tulip
We will not have 1 syntax that does 2 different things, and where you can only tell which one it does by reading the entire rest of the module
It's a relatively common source of bugs in Erlang
ivory ember
this IS possible in Erlang I believe?
viral ivy
case a {
#(x, y) if x == b.0 && y == b.1 -> …
#(x, y) -> …
}
viral ivy
this IS possible in Erlang I believe?
Both erlang and elixir have a pinning operator yeah
ivory ember
right, I assumed that you could do it the same way
oblique tulip
You can define a variable and suddenly a case expression elsewhere in the function stops working
oblique tulip
Both erlang and elixir have a pinning operator yeah
Erlang does not
viral ivy
Oh!
hollow torrent
Shadowing isn’t allowed in erlang right so pinning is out of the box