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[Caml-list] Dynamically evaluating OCaml code
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Date: 2004-04-16 (19:55)
From: skaller <skaller@u...>
Subject: Re: [Caml-list] build tools - good vs. fast, both cheap
On Sat, 2004-04-17 at 04:43, Kenneth Knowles wrote:
> > I'm against make. It just isn't necessary. Make is for C developers,
> > not Ocaml package installation.
> So, in a similar system I'll call "MagicScript," because I'm talking
> hypothetically, not particularly about Interscript:
> * Efficient recompilation:  Use a makefile tangler?  Include a "make"
>   python/OCaml module?  If either of these, then "MagicScript" should provide
>   them.

That's two options. Another is what I actually do in Felix.

    # STEP 4: Compile ocaml interfaces
    for interface in INTERFACES:
      file = 'src/'+interface
      if buildall or filetime(file + '.mli') > filetime(file+'.cmi'):
        buildall = 1
        xqt(OCAMLC + ' -c '+file+ '.mli')

    # STEP 5: Compile ocaml implementations
    for implementation in IMPLEMENTATIONS + EXES:
      file = 'src/'+implementation
      if buildall or filetime(file+ '.ml') \
          > filetime(file+OCAML_OBJECT_EXTENSION):
        buildall = 1
        xqt(OCAMLOPT + ' -c ' + file +'.ml')

This of course is a weaker version of the logic make
implements: it starts compilation at the first module
which is out of date and compiles all the others
after it in the list in order. That's enough for me for Felix.

So there are multiple options .. as many as there are

> * Typical "automake" actions: build, install, uninstall, etc - should be
>   generated from metadata by "MagicScript" so I don't re-invent those wheels.

Sure, if you want! You can always write these scripts yourself,
and just include them. Alternatively, you can add them
to the MagicScript standard distro.

For an Ocaml-centric tool, you'd certainly want to have
a library with Ocaml-centric functions, as well as
generic ones.

Here are some interesting functions:

	let f = open_notouch(filename) in
	let changed = close_notouch(f) in ...

what this does is: the file is opened for INPUT.
Every writeline does a COMPARISON with the input
by reading it. When a line compares not equal,
the file is reopened for OUTPUT and positioned
at the changed line, and each subsequent write
actually writes to the file. The 'changed' flag
determines if the file was touched.

Using these functions whilst generating code
avoids touching unchanged files. This not only
allows 'make' to make the minimal set of things,
it also allows you to determine if your
build process has converged... if every file
written is unchanged, you can stop an iterative
rebuild because you've reached a fixpoint.

If you tell MagicScript to do this:

	ocamlc -c x.mli

it will create a .cmi file. But what if you 
do this instead:

	success = ocamlc -c x.mli -o tmp
	changed = tmp <> x.cmo | not success
	if changed cp tmp x.cmo

With that logic, you might do one extra recompile.
What's the point? The answer is: you don't CARE
what the dependencies are. Ocamlc knows already,
just execute it, either the compile fails,
it succeeds but the output is different to the old one,
or that part of the process has reached a fixpoint.

With that idea .. you can just compile every changed
file in any order, until they're all fixed.

Of course you DO try to optimise the order by trying
to find out the dependencies.. but an unaccounted
for dependency will not mess up the process (usually:
false fixpoints are possible but rare, and of course
cycles are also possible .. don't write the current
date/time into generated output tracked as above!)

Example: new verion of Ocaml installed .. we forgot
that the cmo depends on ocamlc .. (has no effect
on an already fixed system so you need to force
a rebuild in that one case).

Note that this 'build everything repeatedly until it 
all fixes' idea is completely different from make.
It assume a set of *executable build rules*,
and you must track the outputs. But you do not
build 'targets' and you don't need to specify
'dependencies' to find subtargets. There is a single
*universal* target: fixation.

The theoretical underpinings are different to make:
dependencies don't define build order,
they optimise it. The consequences are profound:
the system handles circularities. Make does not.
Almost all non-trivial code generating systems
have circular depenencies, because they invariably
involve bootstrapping (by poetic licence I define
'trivial' as 'not involving bootstrapping' :D

Bottom line: no need to use interscript,
but please lets not just copy existing tools
such as make without doing some theoretical analysis.

John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net

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