Lemmih wrote:
On 5/19/07, Frederik Eaton <[EMAIL PROTECTED]> 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
> 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:
> 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:
> 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:
> 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:
> 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
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