shake-monadic-composition.md

Monadic Build Systems are more Compositional

I've been in favour of builds systems with monadic dependencies for a while. But why. They're more compositional. Imagine:

cat encrypted.txt | rot13 > main.c
gcc -c main.c 

How can we model the dependencies. In Shake we can write:

"main.o" *> \out -> do
    need ["encrypted.txt"]
    cmd Shell "cat encrypted.txt | rot13 > main.c"
    Stdout out <- cmd "gcc -c -M main.c"
    needMakefile out

All easy enough. Writing that same snippet in an applicative build system is difficult. You have two options:

• Guess the include files from encrypted.txt in advance.
• Run it in two phases.

Now it turns out there are almost no purely applicative build systems - they all have slightly more power than that. For example, picking Ninja we can write:

main.o: encrypted.txt
    cmd = (cat encrypted.txt | rot13 > main.c) && gcc -c -M main.c
    deps = afterwards

What I'd argue is that only a monadic build system can be structured so it's compositional. With fairly local changes we can generalise.