Add a variable %{target}

[aalekseyev]

Add a variable %{target} which expands to the one target where unambiguous.

This is to make command lines like these more obviously do what you want and not something crazy:

cmd > %{targets}
tool.exe -o %{targets}

Signed-off-by: Arseniy Alekseyev [email protected]

[ghost]

This is vaguely related to the discussion we had about naming the first dependency. At the time we decided against it given that we have a general syntax for naming dependencies. Here the goal is slightly different and more convincing than the reason to introduce %{first-dep}, so maybe that's ok. /cc people who were involved in the original discussion about first-dep: @rgrinberg @emillon @Chris00.

In any case, I'm thinking that if we introduce %{target}, we should also allow the field name to be just target and only introduce the implicit %{target} variable when (target ...) is used instead of (targets ...), i.e.:

(rule
 (target x)
 (action (... %{target} ...)))

The reason for this is that I'm hoping that we can later formalise the dune language a bit more. Currently it's quite ad-hoc. I don't know yet how that will work, but my feeling is that it will be easier if the name of the implicit variable matches the name of a field.

[aalekseyev]

I thought an extra field target would make the language slightly harder to understand rather than easier, and is more code.
I only weakly think that though. Seems Ok.

[emillon]

I agree with @diml, it seems a good idea not to mix target and targets too much. More generally speaking, automatically casting a list into a space-separated string can be a bit surprising, but it would be difficult to change.

[aalekseyev]

I pushed the version that adds a field target and only lets you use %{target} when the field target is also used.

I also fixed a bug where in the first commit I check uniqueness before expansion, not after expansion.