j/get-in macro v.s j/get-in function about js-interop

[mhuebert]

Hi @lucywang000, thanks for writing. The nuances of macros in ClojureScript took me a long time to grok. The short answer is that a macro can only be placed in the first position of an s-expression, eg. (+ 1 2). A macro needs to be in this position so that it can 'see' the arguments being passed to it (in this case, 1 and 2), as it is a kind of code-rewriting rule. Functions are more versatile - you can see them 'floating around' as values, such that they have no 'literal' visibility of the arguments they will passed - eg. in (reduce + 0 [1 2 3]), the + can have no awareness that it will be used to sum up the numbers 1, 2 and 3 - that happens only at evaluation-time. Other examples would include partial application (partial + 10), usage with map/filter (map + [1 2 3]), composition (comp pos? (partial + 10)).

In ClojureScript we have the ability to bind the same name to both a macro (for situations where it appears in the first position of an expression) and a function (for all other uses). In my experience this is usually for performance reasons; the macro will do a bit of extra work at compile-time, so that less needs to happen at runtime, or can directly emit some efficient javascript. + is an example of something in the standard library that uses this approach.

[lucywang000]

@mhuebert Thanks for the explanation. I never used j/get-in as a function argument, looks like it's very flexible in that way!