今天在编译一个老版的openwrt时，发现有如下的错误。
File "./main.ml", line 777, characters 22-49:
Warning 52: Code should not depend on the actual values of
this constructor's arguments. They are only for information
and may change in future versions. (See manual section 8.5)
File "./main.ml", line 956, characters 35-60:
Error: Unbound module Parmap
Makefile:656: recipe for target 'main.cmo' failed
后发现是ocaml的版本太新了，我的openwrt太古老了。将ocaml降到4.02.3，就可以解决了。
opam switch 4.02.3
eval $(opam config env)
opam install ocamlfind
opam install camlp4