GHC frontend plugin GhcStdin

In GHC, it is not possible to read source code from the standard input.

$ echo 'module Main where main = putStrLn "Ok"' | ghc -o simple_ok
ghc-9.2.3: no input files
Usage: For basic information, try the `--help' option.

This plugin makes this possible.

$ echo 'module Main where main = putStrLn "Ok"' | ghc --frontend GhcStdin -ffrontend-opt="-o simple_ok"
[1 of 1] Compiling Main             ( ghc-stdin-d8c31cf0ed893d79/ghc-stdin260612-0.hs, ghc-stdin-d8c31cf0ed893d79/ghc-stdin260612-0.o )
Linking simple_ok ...
$ ./simple_ok
Ok

Notice that GHC flags are passed via -ffrontend-opt in a single string.

Another use case is collecting exported FFI C functions from a module and putting them in a new shared library.

$ export NGX_MODULE_PATH=/var/lib/nginx/x86_64-linux-ghc-$(ghc --numeric-version)
$ echo 'module NgxHealthcheck where import NgxExport.Healthcheck ()' | ghc --frontend GhcStdin -ffrontend-opt="-Wall -O2 -dynamic -shared -fPIC -flink-rts -threaded -L$NGX_MODULE_PATH -lngx_healthcheck_plugin -o ngx_healthcheck.so" 
[1 of 1] Compiling NgxHealthcheck   ( ghc-stdin-74de48274545714b/ghc-stdin266454-0.hs, ghc-stdin-74de48274545714b/ghc-stdin266454-0.o )
Linking ngx_healthcheck.so ...

(this is a real-world example taken from nginx-healthcheck-plugin).

Internally, the plugin creates a temporary directory with a temporary source file inside it with the contents read from the standard input. Then it spawns another GHC process to compile this file with the options passed in -ffrontend-opt.

Installation of the plugin

To use this frontend plugin with GHC, the built module must be registered as a known GHC package. The simplest way to do it is running

$ cabal install --lib