The spec is correct; your code is wrong and I don't know how you got your
expansion there.
$ lein try org.clojure/clojure 1.9.0
nREPL server started on port 55018 on host 127.0.0.1 - nrepl://
127.0.0.1:55018
REPL-y 0.3.7, nREPL 0.2.12
Clojure 1.8.0
Java HotSpot(TM) 64-Bit Server VM 1.8.0_144-b01
Docs: (doc function-name-here)
(find-doc "part-of-name-here")
Source: (source function-name-here)
Javadoc: (javadoc java-object-or-class-here)
Exit: Control+D or (exit) or (quit)
Results: Stored in vars *1, *2, *3, an exception in *e
user=> (defmulti mycomp (fn [v] v))
#'user/mycomp
user=> (defmacro defcomp [m]
#_=> `(defmethod mycomp ~m
#_=> [_]
#_=> ~m
#_=> )
#_=> )
#'user/defcomp
user=> (defcomp :test)
CompilerException java.lang.RuntimeException: Can't use qualified name as
parameter: user/_, compiling:(null:1:1)
user=> (macroexpand '(defcomp test))
(. user/mycomp clojure.core/addMethod test (clojure.core/fn [user/_] test))
user=>
The problem is that you have not protected the _, so it gets expanded to
the local namespace (user/_ in my case here, com.mxsys.psql.component/_ in
yours). You want:
user=> (defmacro defcomp [m] `(defmethod mycomp ~m [~'_] ~m))
#'user/defcomp
user=> (defcomp :test)
#multifn[mycomp 0x545609d8]
user=> (macroexpand '(defcomp :test))
(. user/mycomp clojure.core/addMethod :test (clojure.core/fn [_] :test))
user=>
Notice how the underscore is escaped as ~'_ to escape from the syntax quote
and place a literal underscore symbol.
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