Re: Dynamically choosing the main function

Harendra Kumar writes:

On 12 November 2017 at 23:18, Ben Gamari wrote:

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In general it's not easy to get a reference to a an arbitrary function in an object file by it's source name. Unless the symbol is exported the compiler is free to optimize the function or even drop it altogether.

I understand that. I was also wondering if there is way (some sort of annotation, pragma or any other workaround) to keep the symbols around without actually exporting them. Exporting either clobbers the module interface from user point of view or the modules need a wrapper to avoid exporting such symbols in user exposed modules. I would also expect an option to remove the effect of any such annotation so that the production build is not under optimized because of this. This is in fact one of the problems that I have been facing in general. I do not know of a good way to hide symbols from users but use them in tests and benchmarks during dev. What is the GHC recommended way to achieve this?

The usual way of dealing with this is to expose the symbols from a `.Internal` module.

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If you can guarantee that the functions you are about are exported then in principle you could probably do what you ask from a library. Just `dlopen` the executable (which would need to be compiled as a position independent exectuable), and dlsym the appropriately mangled name.

Yeah that is a good option for dynamically linked executables. In fact I have been using dlopen to implement Haskell plugins but did not think of using it for this use case. Thanks for reminding me. In a static executable, does it make sense to make the RTS choose an exported symbol as entry point using an RTS option?

I don't see much precedent for this in other compilers. Moreover, I am generally not convinced that it should be the compiler's job to provide such a feature when the same end could be reached with far less magic by the user. Cheers, - Ben