[Haskell-cafe] Extending GHCi

Anthony Cowley acowley at seas.upenn.edu
Fri Feb 4 16:33:41 CET 2011

On Fri, Feb 4, 2011 at 9:41 AM, C K Kashyap <ckkashyap at gmail.com> wrote:
> Thanks Steffen,
>> Prelude> :l MyModule.hs
>> *MyModule> conn <- waitForAndAcceptConnection
>> *MyModule> someData <- getSomeData conn
>> *MyModule> sendSomeAnswer conn $ processSomeData someData
>> ...
> So this cycle of getting data from the connection and writing answer on the
> connection should happen concurrently with the ghci interaction ... so lets
> say that when the "thread" is forked that listens on socket behaves like an
> echo server ... as in, it reads data from the client till "\n" and echoes it
> back ... All this would happen without the intervention of the user using
> GHCI ... However, using GHCI, the user should be able to modify the code
> such that the server returns "hello" prepended to the input. ..
>> startMyServer -- at this point the the echo server gets spawned
>>                       -- echo server continues to run
>> someFunction "hello" --- now onwards  hello gets prepended
>>                       --- echo server continues to run returning "hello"
>> prepended
>> someFunction "world" --- now onwards "helloworld" get
> I hope this is possible without having to modify ghci itself.

Something like this, perhaps. Sorry that it is a bit hard to read.


$ ghci
GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> :m +Data.IORef Control.Concurrent Control.Monad
Prelude Data.IORef Control.Concurrent Control.Monad> msg <- newIORef "Hello"
Prelude Data.IORef Control.Concurrent Control.Monad> let echo =
forever $ readIORef msg >>= putStrLn >> threadDelay 3000000
Prelude Data.IORef Control.Concurrent Control.Monad> t <- forkIO echo
Prelude Data.IORef Control.Concurrent Control.Monad> Hello
writeIORefHello msg "World"
Prelude Data.IORef Control.Concurrent Control.Monad> World

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