Re: capabilities of GHC API

Lemmih wrote:

On 5/19/07, Frederik Eaton wrote:

...

Hello,

I think what I'm trying to do is too ambitious, but I thought I would ask to see if it is possible. It seems like there is no easy way to do it, given what I've seen of the GHC API.

I would like to have a function, say it is called "this", which has the following effect in ghci

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let n = 2 in this n 2

In other words, it captures all the variables which are in scope, and adds them to the GHCi environment. Somebody helpful will probably say "But you can just write 'let n = 2'!", but that is not the aim. There are several aims. One is to be able to look at the variables inside a function which one is trying to debug, then inserting 'this' will cause them to be in scope, I think that would be useful. A more important aim is to be able to use existentially quantified variables easily. Currently I can do:

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reifyIntegral 5 (\n -> print $ reflectNum n) 5

but how can I get GHCi to have an 'n' binding which is inside the function? Clearly just returning 'n' will not work:

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reifyIntegral 5 id

<interactive>:1:0: Inferred type is less polymorphic than expected ...

This is what I am thinking of doing, but as I said it seems ambitious. There are several easier things one could think of:

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let n = 2 in bind "n" n n 2

If it were possible to add bindings to the GHCi bindings list, then this would be easy. Is it possible? The documentation doesn't seem to mention such a capability.

Also, probably another useful feature would be to combine 'this' with something in the IO monad:

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withProgName "blah" thisIO getProgName "blah"

So, are these things currently possible? Planned? Have the functions I describe been implemented already? I think there is a GHCi debugger in the works, so maybe functionality like this will be part of it, I didn't want to start something on my own if that is the case...

This is pretty much that the GHCi debugger does expect it restores the environment at the end of a breakpoint.

If you have GHC-6.6 or greater, try: let n = 2 in GHC.Base.breakpoint ()

GHC.Base.breakpoint doesn't work in the HEAD at the moment. We might want to restore it; I'm not sure. Since breakpoints are almost everywhere, it didn't seem necessary. I don't currently have breakpoints in either compiled code or expressions typed at the prompt, so you can't do exactly what Frederik was asking for, although if the code is in a source file then it works fine. Instrumenting expressions typed at the prompt wouldn't be hard, I'll add that to the list. The documentation for the GHCi debugger is here, FYI: http://www.haskell.org/ghc/dist/current/docs/users_guide/ghci-debugger.html Cheers, Simon