Clojurescript macros: Using the node api at compile time

Question

I'm writing a cli script with lumo and I want the following macro, but using readFileSync from nodejs.

(defmacro compile-time-slurp [path]
  ;; slurp is not defined in self hosted cljs
  (slurp path))

Is this possible?

EDIT: To be more clear, this is in self-hosted clojurescript, where the slurp function is not available at macro-expansion-time.

amalloy · Accepted Answer · 2017-07-07 00:03:22Z

4

ClojureScript macros are written in the Clojure language and have roughly this lifecycle:

jvm loads Clojure runtime, gets ready and a bunch of other stuff.
macro is compiled
macro runs and produces a new ClojureScript expression
if that expression is a macro loop again.

This omits all the parts done in the rest of the ClojureScript compiler (which is most of it) so we can focus on the fact that ClojureScript macros only have access to the parts of Clojure that are available from the JVM and not node while they are running the form returned by that macro which will become part of the finished ClojureScript program has access to the node APIs such as readFileSync.

In short, your macro should return a call to readFileSync rather than do the file reading while the macro is running. If your code really needs to read some files while evaluating the macro, because for instance they contain the code to output or something, then you would need to use the Clojure form to read those files such as the call to slurp you have above.

edited Jul 7, 2017 at 0:03

amalloy

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answered Jul 6, 2017 at 23:12

Arthur Ulfeldt

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3 Comments

Arthur Ulfeldt Over a year ago

Thanks again amalloy for fixing my speelng and gramarrrr :-)

madstap Over a year ago

Maybe it wasn't that clear in the question, but I'm using lumo, which is self-hosted. And in self-hosted the slurp function doesn't work at compile-time.

Arthur Ulfeldt Over a year ago

it's not clear how to make this work with these constraints, sorry :-/ I hope there is some way to can avoid using a macro here and dodge this problem. These things usually come up due to "macro contagion" and can be a pain.

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