Like 'make' except:
 - Predicates can be based on arbitrary expressions, not just
   "file X is older than file Y".
 - Rules are more flexible and encourage structuring and reuse
   through functions.
 - Goals can be parameterized.
 - Program can run continuously to implement business rules.

Differences from 'whenjobs':
 - Goals instead of variables.
 - Persistent (across session) variables exist, but are not central.
 - Doesn't use <<..>> for shell scripts (has a function 'sh' instead).

Similarities to 'whenjobs':
 - Each shell script runs in its own temporary directory.

Example Makefile rule and translation to goaljobs language:

  %.o: %.c
    cc -c $< -o $@

  let goal compile c_file =
    require (file_exists c_file);
    let o_file = replace_substring ".c" ".o" c_file in
    target (file_exists o_file && file_newer o_file c_file);
    sh "cc -c %s -o %s" c_file o_file

Example program:

  let package = "foo"

  let rec goal website_updated version =
    let tarfile = sprintf "%s-%s.tar.gz" package version in
    let tarpath = getenv "HOME" // "html" // tarfile in
    let url = sprintf "http://example.com/%s" tarfile in

    target (url_exists url);

    require (tarball_exists version);
    require (tarball_tested version);

    sh "rsync %s example.com:/html/" tarpath

  and goal tarball_tested version =
    let tarfile = sprintf "%s-%s.tar.gz" package version in
    let tarpath = getenv "HOME" // "html" // tarfile in
    let memkey = package ^ "_tested_" ^ version in

    target (memory_exists memkey);

    require (tarball_exists version);

    sh "
      tar zxf %s
      cd %s-%s
      ./configure
      make
      make check
    " tarpath package version;

    memory_set memkey "1"

  and goal tarball_exists version =
    let tarpath = getenv "HOME" // "html" // tarfile in
    target (file_exists tarpath);
    sh "
      cd $HOME/repos/%s
      git fetch
      git archive --prefix %s-%s/ v%s | gzip > %s-t
      mv %s-t %s
    " package package version version tarpath tarpath tarpath

  every 1 hour =
    let version = shout "
      cd $HOME/repos/%s
      git fetch
      git describe --tags --abbrev=0 --match='v*'
    " package in
    require (website_updated version)