I'm new to nim. I use the Option type in other languages and was playing around
with them in nim (version 2.0.2):
{.experimental: "caseStmtMacros".}
import fusion/matching
let a =
case some(0)
of Some(_): 1
else: 2
echo a
let b =
case some(3)
of Some(_): 4
of None(): 5
echo b
Run
I get a compile error for line 13 `case some(3)`:
> template/generic instantiation of `case` from here
and for line 14 `of Some(_): 4`:
> Error: expression '4' is of type 'int literal(4)' and has to be used (or
> discarded)
I am trying to understand this. My guess is that since there is no `else`
clause in the `case some(3)` statement, nim infers the type of this statement
as the absence of any type. It then expects all branches of the `case some(3)`
statement to have no type. So then nim complains that the `4` has to be
discarded in order for the `Some(_):` branch to have no type.
The reason [this
example](https://nim-lang.org/docs/options.html#pattern-matching) works is
because both branches have `assert` statements that have no type.
So nim doesn't know that the `Some(_)` and `None()` branches form an exhaustive
check of Option values? Is that a consequence of the Option type being
implemented as a macro and not built into the language?
Thanks for reading!
