Version 2.16.0, Git revision 1817292 (10654 commits) PRE-RELEASE x86_64 has --haddock-executables (built on top of Cabal (the library)'s haddock --executables), but there are problems with the index.

Consider a two-package project (packageA, packageB) each with two executable components (eg packageA-exe1, packageA-exe2). The index generated for project packages has four links to Main but all of them are to the Main of packageB-exe1.

Using haddock --show-interface reveals that all four *.haddock files commence:

{"link_env":{"$main$Main$main":"Main", ...

That can be contrasted with the two *.haddock files for the main library:

{"link_env":{"$packageA-0.1.0.0-ErEp6QMCDA5L7wclG18d4W$Lib$someFunc":"Lib"}, ...

and

{"link_env":{"$packageB-0.1.0.0-LJmkqLQIxknEQ7YIpA65Ah$Lib$someFunc":"Lib"}, ...

--cabal-verbosity 3 reveals that, with the main library, Cabal (the library) invokes Haddock with options that include:

packageA> --optghc=-this-unit-id
packageA> --optghc=packageA-0.1.0.0-ErEp6QMCDA5L7wclG18d4W

packageB> --optghc=-this-unit-id
packageB> --optghc=packageB-0.1.0.0-LJmkqLQIxknEQ7YIpA65Ah

There is no equivalent for when Cabal invokes Haddock for the executable components.