apply macros to a vector

Question

Imagine having a dynamically generated specs for cat

[:first-name string? :surname string?]

Now I want to use them with cljs.spec.alpha/cat.

In plain Clojure I can write a macro that will cons a macro with args and eval it, right? But for ClojureScript it is not as easy because macros are compile-time beasts and also eval is also kinda different thing.

What are workarounds to apply my vector of args to a macro in ClojureScript?

I think I can't do it, so I'll go using cat-impl directly. — fl00r
– fl00r, Commented Aug 31, 2018 at 12:33
Is the vector defined at compile-time or will it be dynamically constructed at runtime? — olieidel
– olieidel, Commented Aug 31, 2018 at 16:49
@olieidel it will be defined at compile time (see github.com/fl00r/trout) — fl00r
– fl00r, Commented Aug 31, 2018 at 17:03

Taylor Wood · Accepted Answer · 2018-08-31 12:58:24Z

2

Unfortunately this isn't easy for dynamic/data-driven specs, but that's apparently being worked on for a future release.

I think the only workarounds are to eval, or use cljs.spec.alpha internal implementation as you mentioned.

cljs.user=> (def cat-args [:x string?]) #'cljs.user/cat-args cljs.user=> (s/def ::foo (eval (cons 's/cat cat-args))) :cljs.user/foo cljs.user=> (s/conform ::foo '("foo")) {:x "foo"}

edited Aug 31, 2018 at 12:58

answered Aug 31, 2018 at 11:42

Taylor Wood

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4 Comments

fl00r Over a year ago

Oh, sorry, I am talking about cljs env, not clojure one. you don't have args on compile time, and macros are not runtime in cljs

fl00r Over a year ago

are you using cljs.core/eval? because it shouldn't work as you wrote unless you redefined *eval* (cljs.github.io/api/cljs.core/STARevalSTAR)

Taylor Wood Over a year ago

@fl00r yes, I just tried (cljs.core/eval (cons 's/cat cat-args)) and it works. FWIW I'm doing this in a Planck REPL.

fl00r Over a year ago

seems like a decent tool

Mike Fikes · Accepted Answer · 2018-08-31 21:26:04Z

Is your spec generated symbolically at compile time? If so, perhaps you could use a macro as you had suggested.

(ns foo.core) (defmacro gen-spec [] `'~(into '[:first-name string?] '[:surname string?])) (defmacro apply-macro [name args-form] `(~name ~@(eval args-form)))

Here is an example using it

$ clj -m cljs.main ClojureScript 1.10.339 cljs.user=> (require '[clojure.spec.alpha :as s]) nil cljs.user=> (require-macros 'foo.core) nil cljs.user=> (foo.core/gen-spec) [:first-name string? :surname string?] cljs.user=> (foo.core/apply-macro s/cat (foo.core/gen-spec)) {:cljs.spec.alpha/op :cljs.spec.alpha/pcat, :ps [#object[cljs$core$string_QMARK_] #object[cljs$core$string_QMARK_]], :ret {}, :ks [:first-name :surname], :forms [cljs.core/string? cljs.core/string?], :rep+ nil}

Note how gen-spec builds up the spec symbolically and returns it in quoted form.

Thank you Mike for your feedback. Unfortunately the specs themselves are being generated from another data structure in compile time. So I don't have them in a ready to use form. Look an example in Readme (github.com/fl00r/trout). So I can't use your approach.

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