Maelstrom and make

It should be obvious by now that Maelstrom uses a very different execution algorithm than make. First, Maelstrom presumes that all of the scripts it controls use exit codes to indicate whether they should be repeated. In make, a non-zero exit code instead indicates an irrecoverable error and causes a full stop. The most crucial difference, though, is that make utilizes centralized or global knowledge of precedence in order to schedule its tasks, while Maelstrom's knowledge is a local result of the behaviors of the scripts it controls. This means that to emulate make with Maelstrom, some of the tasks done by make must move into the scripts that Maelstrom dispatches.

For example, consider the trivial Makefile:

foo: foo.o bar.o
        g++ -o foo foo.o bar.o 
foo.o: foo.c
        g++ -c foo.c
bar.o: bar.c
        g++ -c bar.c

This describes how to make an executable foo from source files foo.c and bar.c. To simulate this in Maelstrom, we break it into three convergent tasks c1, c2, and c3: one for each compilation command in the Makefile. The convergent process of creating foo.o from foo.c is easy to express in the (shell) script c2:

if [ -nt foo.o foo.c ]; \
  exit 0
g++ -c foo.c
exit 0

(-nt x y is true if x is newer than y). Likewise, compiling bar.c into bar.o can be accomplished in a script c3:

if [ -nt bar.o bar.c ]; \
  exit 0
g++ -c bar.c
exit 0

Difficulties, arise, however, in performing the final step of creating foo from foo.o and bar.o. In make, the states and dates of all files are known, and this global knowledge is used to assure that each command operates upon up-to-date data. Maelstrom knows none of this, so we must compensate for its lack of awareness by checking all prerequisites inside the compilation script before doing the final compilation:

if [ -nt foo.c foo.o ]; \
  exit 1
if [ -nt bar.c bar.o ]; \
  exit 1
if [ -nt foo foo.o \
  -a -nt foo bar.o ]; \
  exit 0
g++ -o foo foo.o bar.o 
exit 0;

The script that combines all results must compensate for Maelstrom's lack of global knowledge by obtaining that knowledge for itself. If we call this script c1, then the Maelstrom configuration:

c1 : c2 
c1 : c3

has the same result as the above Makefile.

This may seem like a limitation until we realize that in Maelstrom's problem domain, the kind of global knowledge that make utilizes is typically unavailable or approximate. Since troubleshooting scripts, for safety, must check that their prerequisites are satisfied before continuing, Maelstrom's lack of global knowledge does not impact its ability to solve troubleshooting problems.