Why cabal tool doesn't use Setup.lhs/Setup.hs?

Question

I've added a putStrLn "Hello" line into main function of my Setup.lhs and was expecting to see it when running cabal configure or cabal build. But i did not.

Then i've compiled Setup.lhs with ghc --make and ran ./Setup configure and the line was shown.

If it's done intentionnaly, i don't see rationale behind this and even need in Setup.lhs file at all. Can you clear these things for me?

Answer 1

You most likely have

build-type: Simple

in your .cabal file. If you select the Simple build type, you essentially promise that your Setup file does nothing but invoke defaultMain, and the cabal binary will not invoke it. If you want to ensure that your Setup file is run every time, then change the line to

build-type: Custom

You also ask about the rationale for requiring the Setup file anyway: actually, it isn't required if you use the Simple build type. The cabal binary will happily configure and install it without. However, it is considered good style to include a Setup file for any package, because it will allow users to install the package who have the Cabal libary available, but not the cabal-install tool (and Hackage enforces the presence of a Setup file for this reason).