[Haskell-cafe] executable scripts?

frederik at a5.repetae.net frederik at a5.repetae.net
Sat Oct 23 04:42:01 EDT 2004

> One of the nice things about perl (for example) is that you can put
> together a script with #!/usr/local/perl (in bash for example) as the first
> line of a file and run it immediately.  I've used perl a lot this way with
> simple 'throw away' scripts to do special filtering on a file, or some
> other processing that I want to do one or a few times.  occasionally, a
> script like this will have a more permanant value to me, and I keep it
> around.
> Is there some way to do something similar in with Haskell?   I've tried the
> most obvious (to me) test with Hugs and it doesn't work.
> Surely there is a way to do this!

I've been using an interpreter script I wrote which just uses ghc to
create an executable in /tmp, and then runs it. It compares
modification times, so if your script hasn't changed, it doesn't run
ghc and there isn't much overhead. Thus it is faster than runghc in
the common case. The thing is, it doesn't work very well. If your
script depends on a module, and that module changes, then it doesn't
know to recompile the script.

I tried to get around this using "ghc --make", but unfortunately this
isn't as fast as it could be in the common case (I think it could be
as fast as "ghc -M", but isn't), and is therefore unsuitable for our
application. I tried using "ghc -M", "make -q" and then "ghc --make",
but it appears that "ghc -M" outputs dependencies on .hi files whose
modification times aren't always updated by "ghc --make" when they are
rebuilt, causing a stuck "out-of-date" condition which breaks the
dependency analysis.

Anyway, my interpreter script is here, but at least in part because I
wasn't able to get around the above problems, it isn't very good:



$ cat test
#!/usr/bin/env hs-interp
main = do putStrLn "hello world"
$ ./test  
hello world

-- Frederik

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