GHC frontend plugin GhcStdin
============================

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

```ShellSession
$ 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.

```ShellSession
$ 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.

```ShellSession
$ export NGX_MODULE_PATH=/var/lib/nginx/x86_64-linux-ghc-9.2.3
$ echo 'module NgxHealthcheck where import NgxExport.Healthcheck ()' | ghc --frontend GhcStdin -ffrontend-opt="-Wall -O2 -dynamic -shared -fPIC -lHSrts_thr-ghc$(ghc --numeric-version) -L$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](https://github.com/lyokha/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*. Note that the options get collected by *words* without
passing them to a shell preprocessor which means that it is not possible to
escape spaces in their values with quotes or backslashes.