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From: Alexy Khrabrov <deliverable@gmail.com>
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Date: Thu, 14 Apr 2011 19:32:16 -0400
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Subject: [Caml-list] swapping variant modules via subdirectories fails at link time

I have a module a.ml, containing some functions I want to alter in a version of my program.  I'd like to do it on the command line at link time, to obtain either the original, or an altered version.  

Here's the original make line, simplified:

%.cmx: %.ml
	ocamlfind ocamlopt $(DEBUG) $(OPTFLAGS) -package $(PACKAGES) -c $^ -o $@

prog.opt: lib.cmxa a.cmx x.cmx prog.ml
	ocamlfind ocamlopt $(DEBUG) $(OPTFLAGS) -package $(PACKAGES) -linkpkg $^ -o $@

I tried to create the variant by placing an altered version of a.ml in a subdirectory, var/, and linking that:

prog.var.opt: lib.cmxa var/a.cmx x.cmx prog.ml
	ocamlfind ocamlopt $(DEBUG) $(OPTFLAGS) -package $(PACKAGES) -linkpkg $^ -o $@

Further, I've created a.mli at the root level, and compiled it with

ocamlfind ocamlc a.mli

-- it needs lib.cmo to work, a byte-code version of lib.cmxa, which I make first.

With that a.cmi at the root level, I can remake prog.opt fine.  If I copy or symlink a.cmi to var/, or symlink a.mli there, and try to make prog.var.opt, I get an error at prog.ml that files var/a.cmx and x.cmx make inconsistent assumptions about the interface A.

The var/a.cmx is made by the same above pattern rule as ./a.cmx.  Even when ./a.cmi and var/a.cmi are the same, the same inconsistent error shows up!  Renaming ./a.ml out of the way and symlinking var/a.ml to ./ remakes prog.opt fine, which I can then rename prog.var.opt manually and hope to do it every time I need that, yet it's tiresome.   What prevents the original scheme from working?

-- Alexy