Warn about threading macro with empty forms argument

[serioga]

Is your feature request related to a problem? Please describe.

Sometimes during refactoring my threading macro invocations reduce to the form like (-> 'x).
In this case the macro has no effect and can be removed.

Describe the solution you'd like

I would like to get linting warning when threading macro is useless and can be removed.
This is about all macros ->, ->>, cond->, cond->>, some->, some->>, as-> and doto when forms argument is empty.

[borkdude]

There might be more functions that just return the argument when called with 1 argument so possibly we could generalize this a bit.

We already have:

:single-operand-comparison {:level :warning}
:single-logical-operand {:level :warning}

The difference between the first and the latter is that the first always returned true and the second one always returned the argument (e.g. (and x), (or x)). The second linter may be merged with this case where it's just an redundant identity op.

So maybe :redundant-identity-call is a good name? Open for better suggestions.

[serioga]

English is not my native language...
But :redundant-identity-call sounds bad for me: a) identity associates with core identity b) for macros it is not a real “call”. Maybe something like :redundant-form-usage...

[serioga]

Or just :redundant-form :-)

[borkdude]

A function always returning the same output as its input is called an identity, this is exactly what identity does.
-> is a macro which just returns the form given, when called with one argument.

I think :redundant-form isn't good since that would suggest the entire form would be redundant, while only the called function is redundant.

Maybe :redundant-call (both for macros and functions) is better. Let's think about it some more. Naming is hard.

[serioga]

I suggest to ask in slack :-)

[serioga]

I think :redundant-form isn't good since that would suggest the entire form would be redundant, while only the called function is redundant.

I thought it will sound in practice like :redundant-form -> so it's clear what form is it about :-)

[serioga]

:ineffective-invocation :-)

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[borkdude]

Already implemented:

$ clj-kondo --lint - <<< '(-> 1)'
<stdin>:1:1: warning: Single arg use of clojure.core/-> always returns the arg itself
linting took 10ms, errors: 0, warnings: 1