[Well-Typed Blog] Making GHCi compatible with multiple home units

[fendor]

@chreekat

Is there any budget for adapting Stack to benefit from multiple home units?

There is currently no budget planned to adapt stack to use GHCi’s multi home unit feature.
However, it would be great to teach stack about multiple home units and will try to answer any question that arises.

@mpilgrem

This may be typographical: How does ghc --interactive -this-unit-id lib-mhu-example ... create a home unit graph with one of the home units being mhu-example-library? I was expecting that home unit to be named lib-mhu-example.

Thanks for pointing this out, this name is a mistake and it should be lib-mhu-example.

@mpilgrem

Also, in the related diagram, the home unit is named lib:mhu-example not lib-mhu-example. I am confused: what is the name of the home unit that is being created?

The name of the home unit is specified by the tool, e.g. cabal invokes GHC with the argument: -this-unit-id ... which is used as the name of the home unit. In the diagram, we took the liberty to use the component name, as users are usually more familiar with components than units.
To be precise, the home unit graph would only use what is given in the -this-unit-id flag. We will not change the format of unit ID, so lib:mhu-example is not a valid unit id. It is only used as an attempt to make it more approachable.

@mpilgrem

What are the arguments that need to be/may be included in each response file specified by the -unit option? In particular, how does the response file for one home unit specify that the unit has a dependency on another home unit? Is it by using the -package-id <unit-id> option? (I have seen the GHC 9.12 documentation on ‘the normal arguments’.)

Yes, precisely, the flag -package-id <unit-id> is used to express the dependency.

@mpilgrem

I also have a third question about Cabal. The post refers to Cabal (the tool). Cabal (the library) also has a runhaskell Setup.hs repl command (although it is not documented in the Cabal User Guide). Am I correct to understand that this functionality is not coming to Cabal (the library)?

The blog post is particularly talking about GHCi support, independent of Cabal, cabal-install or stack.
I am not aware of any efforts of updating Setup.hs repl to use multiple home unit with GHCi.

@mpilgrem

Does it matter in what order the -unit options appear on the command line?

No, it does not!

@mpilgrem

Related to that, if you have GHC flags/options in the various response files and other GHC falgs/options on the command line (before or after -unit options), how does that ‘work’ in terms of what gets applied to what?

Currently, we first parse all cli arguments into DynFlags, including -unit arguments. We then use the initial DynFlags (e.g., the flags given at the top-level) and then process arguments in the -unit response files.

As an example:

ghc -fwrite-ide-info -unit @a -unit @b

then the option -fwrite-ide-info is prepended to the arguments given in @a and @b.
Perhaps counter-intuitively, the order doesn’t matter.

ghc -unit @a -unit @b -fwrite-ide-info 

is the same ghc invocation, the -fwrite-ide-info option would be prepended to the options @a and @b.