External plugins

GHC supports two kinds of "static" plugins: 1. internal: setup with GHC-API 2. external: setup as explained below and loaded from shared libraries

The intended use case for external static plugins is with cross compilers: at the time of writing, GHC is mono-target and a GHC cross-compiler (i.e. when host /= target) can't build nor load plugins for the host using the "non-static" plugin approach. Fixing this is tracked in #14335. If you're not using a cross-compiler, you'd better use non-static plugins which are easier to build and and safer to use (see below).

External static plugins can be configured via the command-line with the -fplugin-library flag. Syntax is:

• fplugin-library=⟨file-path⟩;⟨unit-id⟩;⟨module⟩;⟨args⟩

Example: -fplugin-library=pathtoplugin;package-123;Plugin.Module;[Argument,List]

Building the plugin library: 1. link with the libraries used to build the compiler you target. If you target a cross-compiler (stage2), you can't directly use it to build the plugin library. Use the stage1 compiler instead.

2. if you use cabal to build the library, its unit-id will be set by cabal and will contain a hash (e.g. "my-plugin-unit-1345656546ABCDEF"). To force the unit id, use GHC's `-this-unit-id` command line flag: e.g. -this-unit-id my-plugin-unit You can set this in the .cabal file of your library with the following stanza: `ghc-options: -this-unit-id my-plugin-unit`
3. To make your plugin easier to distribute, you may want to link it statically with all its dependencies. You would need to use `-shared` without `-dynamic` when building your library.

However, all the static dependencies have to be built with `-fPIC` and it's not done by default. See https://www.hobson.space/posts/haskell-foreign-library/ for a way to modify the compiler to do it.

In any case, don't link your plugin library statically with the RTS (e.g. use `-fno-link-rts`) as there are some global variables in the RTS that must be shared between the plugin and the compiler.

With external static plugins we don't check the type of the plugin closure we look up. If it's not a valid Plugin value, it will probably crash badly.