Feature request: direct support for ccalls (like ghcjs)

[jneira]

Description

• eta doesn't support foreign import ccall so we have to patch the code replacing them with foreign import java
• ghcjs supports them converting the ccall calling a javascript function using a name convention: https://github.com/ghcjs/ghcjs/blob/master/doc/foreign-function-interface.md#emulating-c-code-with-foreign-import-ccall
• patching packages would be easier cause in many cases we only would need to add java-sources with the appropiate classes and methods

  Expected Behavior

• eta should translate foreign import ccall generating java calls to class eta.. ( f.e. eta.unix.SystemPosixFiles) and method (rename)
• maybe we could (de)activate the behaviour with a flag
• maybe we could make a tool (integrated in eta/etlas?) to generate auto java stubs with all ccalls of a package using this name convention

  Actual Behavior

• eta rejects foreign import ccall

[rahulmutt]

I like the GHCJS approach. It's a bit implicit, but I think it's OK. The only issue I can see is that a lot of times you need to replace Ptr with a Java object and the actual type signatures change. In the other cases it would be OK.