tuple destructuring doesn't always work with `_` (underscore)

[a-mr]

Tuple desctructuring does not work with underscores if used outside of var section. Inside it it's OK, and that's confusing. It seems Nim sees _ as a special identifier that should be declared.

Example

var x = 0
(x, _) = (1, 2)

Current Output

Error: undeclared identifier: '_'

Expected Output

To work the same way as

var (x, _) = (1, 2)

(This was fixed at some stage, e.g. bug #2421)

Possible Solution

Additional Information

Checked with Nim 1.4 and Nim 1.5 devel.

[rockcavera]

By Nim's definitions, a variable couldn't even start with "_". Look here.

So, for me, the bug occurs in this situation: var (x, _) = (1, 2), as it compiles without error.

[konsumlamm]

It seems Nim sees _ as a special identifier that should be declared.

Not really, it's special, but assigning to it discards the value (so you can't really declare it), you can't use it in an expression or anything.

So, for me, the bug occurs in this situation: var (x, _) = (1, 2), as it compiles without error.

That doesn't make any sense. _ is a special name, it's used to discard a value, for example the following (intentionally) works:

let _ = 42
# the following doesn't compile, since _ isn't a normal variable
# echo _

Now arguably (x, _) = (1, 2) shouldn't work, because _ = 42 doesn't work either, although one could argue that both should be valid. Fwiw, in Rust, this is called "destructuring assignment", but is currently unstable (see the RFC).

[rockcavera]

It seems Nim sees _ as a special identifier that should be declared.

Not really, it's special, but assigning to it discards the value (so you can't really declare it), you can't use it in an expression or anything.

So, for me, the bug occurs in this situation: var (x, _) = (1, 2), as it compiles without error.

That doesn't make any sense. _ is a special name, it's used to discard a value, for example the following (intentionally) works:

let _ = 42
# the following doesn't compile, since _ isn't a normal variable
# echo _

Now arguably (x, _) = (1, 2) shouldn't work, because _ = 42 doesn't work either, although one could argue that both should be valid. Fwiw, in Rust, this is called "destructuring assignment", but is currently unstable (see the RFC).

Sorry, I didn't know the "_" functionality described here.

[bung87]

var (x, _) = (1, 2) is called "tuple unpacking". It seems the issue fit a feature request like @konsumlamm said, it's "destructuring assignment", maybe change a better title.

[metagn]

Won't make the PR since it's new behavior but the patch for this is just skipping underscores here:

Nim/compiler/lowerings.nim

Lines 141 to 142 in e68a6ea

	for i in 0..<lhs.len:
	result.add newAsgnStmt(lhs[i], newTupleAccessRaw(tempAsNode, i))