[UX] ghc seems to parse flags in a way that the first one wins, not last one.

[angerman]

While trying to convince GHC to set -this-unit-id to I learned that passing -this-unit-id multiple times to GHC will result in GHC using the first occurrence in the passed arguments, rather than the last. Thus any ghc-options: set in the cabal file, or even in the cabal.project file, which will be passed later do not have any effect.

ghc -this-unit-id X -this-unit-id Y -this-unit-id Z

will result in GHC using X.

[angerman]

$ ghc -shared -dynamic '-dynload deploy' -optl-Wl,-rpath,/home/andrea/.ghcup/ghc/9.8.4/lib/ghc-9.8.4/lib/x86_64-linux-ghc-9.8.4 -this-unit-id abc-iog -hide-all-packages -no-auto-link-packages -no-user-package-db -package-db dist/package.conf.inplace -package-id base-4.19.2.0-2a19 dist/build/MyLib.dyn_o -o dist/build/libHSabc-iog-ghc9.8.4.so -this-unit-id def-iog

yields

head dist/package.conf.inplace/abc-iog.conf
name:                 cabal-shit-1qy6j
version:              0.1.0.0
visibility:           public
id:                   abc-iog
key:                  abc-iog
maintainer:           [email protected]
author:               Andrea Bedini
abi:                  inplace
exposed:              True
exposed-modules:      MyLib

as @andreabedini verified.