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I decided to switch my compilation environment from a Makefile and a configure file to Oasis.

In my configure file I had this command :

EXTERNALLIB=""
if test "$USEOCAMLFIND" = yes; then
  EXTERNALLIB=$(ocamlfind query -i-format External)
fi

and in my Makefile :

externalwrapper.ml: externalwrapper_actual.ml externalwrapper_fake.ml config.status
    @rm -f externalwrapper.ml
    @if [ -z "$(EXTERNALLIB)" ]; then \
    cat externalwrapper_fake.ml >> externalwrapper.ml;\
    else \
    cat externalwrapper_actual.ml >> externalwrapper.ml;\
    fi;\
    chmod -w externalwrapper.ml

which allowed me to compile an actual Externalwrapper module if the library existed in my computer and a fake one if it didn't.

Now, I try to do it in Oasis but I don't know if it's possible and, if yes, how should I do it ?

Lhooq
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  • I think that this should help: http://stackoverflow.com/questions/39796986/optional-dependencies-with-ocamlbuild/39797476#39797476 – ivg Feb 07 '17 at 15:59
  • Well, according to the answers in this post, what should I do ? Use drup's solution or yours ? ;-) – Lhooq Feb 07 '17 at 16:09
  • he-he, it's your call :) Let me summarize your options in the answer, as I started to get of the space, when I tried to say in the comment. – ivg Feb 07 '17 at 16:50

2 Answers2

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At the time of writing OASIS 0.4.8 supports neither optional nor conditional linking. Although you can build targets optionally, a recipe for the target should be static. There are rumors, that OASIS author is planning to add this feature in 0.4.9, but currently, he is blocked by an issue in ocamlbuild.

There are few workarounds, however.

Preprocessing

You can just add an extra layer of indirection, and build your _oasis file froms some _oasis.in file. You can even use autoconf for this, e.g.,

AC_INIT([Name], [0.1-beta], [your@email])
AC_CONFIG_FILES([_oasis])  

EXTERNALLIB=$(ocamlfind query -i-format External)
WRAPPER_MODULE=externalwrapper_fake.ml
if test -z "$(EXTERNALLIB)"; then
    WRAPPER_MODULE=externalwrapper_actual.ml
fi

AC_OUTPUT
AC_SUBST(WRAPPER_MODULE)

And in the _oasis.in you can use @WRAPPER_MODULE@ variable that will be actualized to externalwrapper_actual.ml or externalwrapper_fake.ml.

Instead of using autoconf substitutions you can actually build your own configure script that will construct _oasis file. For example, in BAP we are constructing a final _oasis file by gluing it from pieces in our custom configure script.

Use Object section and extend OCamlBuild

An alternative solution was proposed by Drup (see the following PR as an example). The idea is to describe each variant as a conditionally built object, and then extend OCamlbuild with a special build rule, that will, depending on a flag, find this object and link it directly.

Both solutions have their cons and pros, so we are all looking forward for the feature to be added into OASIS.

ivg
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  • I'll try it tomorrow and let you know bug man, I love this kind of answers ! If I could give +10 to it I'd do it ! Thanks ! – Lhooq Feb 07 '17 at 17:24
  • Well, I got a problem with what you wrote, I had to replace the `if` with `if test -z "$EXTERNALLIB"` because it was telling me `-z: command not found`. But it works perfectly, thanks ! – Lhooq Feb 08 '17 at 12:29
  • yeah, forgot that brackets are interpreted by the m4 preprocessor. Will fix it. – ivg Feb 08 '17 at 13:21
  • This is completely unrelated but would you know why, when I execute `oasis setup` I have this error : `W: No replace section found in file configure` ? – Lhooq Feb 08 '17 at 14:19
  • Probably, this is because you enabled the DevFiles plugins, and it tries to overwrite your configure file. I would suggest to disable the plugin (you can keep the generated Makefile though, it doesn't depend on the _oasis file). – ivg Feb 08 '17 at 14:22
  • Great! That was, indeed, the problem! – Lhooq Feb 08 '17 at 14:51
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One possibility is to use flags with the name of your library and ask opam to enable them if the library is present (oasis2opam does that for you). See for example the definition of a lwt flag and its use to conditionally build a library. You can also have setup.ml enable the flag if the library is present. Finally, you can add cppo rules in myocamlbuild to enable conditional code compilation.

ChriS
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