[Haskell-cafe] Interpreting synchronized streams (pre-IO monad Haskell)

[Alexis King]

You want unsafeInterleaveIO. That will allow you to build the list of responses as a list of thunks, each of which reads a request and yields a new response when forced.

> On Feb 13, 2020, at 06:10, Jiri Jakes <jiri at jirijakes.eu> wrote:
> 
> Heλλo!
> 
> Before introduction of IO monad, first versions of Haskell used
> synchronised streams [1]:
> 
>  main :: [Response] -> [Request]
> 
> where Response and Request were ADTs containing all possible actions
> and responses:
> 
>  data Request = ReadFile Name | ... | AppendChan Name String | ...
>  data Response = Success | Str String | Bn Bin | Failure ...
> 
> 
> I wanted to write an interpreter of this representation using a
> simplified version:
> 
>  import System.Environment (getArgs)
> 
>  data Response = Success | StrList [String]
>  data Request = GetArgs | PutStr String
> 
>  eval :: Request -> IO Response
>  eval GetArgs = StrList <$> getArgs
>  eval (PutStr s) = putStrLn s >> return Success
> 
>  main' :: [Response] -> [Request]
>  main' ~(StrList args : _) =
>    [ GetArgs,
>      PutStr (show args)
>    ]
> 
>  main :: IO ()
>  main = ???
> 
> I can imagine how interpreting main' could work, thanks to laziness.
> However, when actually trying to implement it, I get lost. I cannot
> figure out how could I read requests (result of calling main') before
> having something to pass to main'. Having a look at ancient version of
> GHC [2] didn't help either.
> 
> Is it actually possible without some low-level tricks? Perhaps using
> mutually recursive functions? Or by converting to continuations somehow?
> 
> Thank you!
> 
> Jiri
> 
> 
> [1] http://haskell.org/definition/haskell-report-1.0.ps.gz
> [2] https://downloads.haskell.org/~ghc/0.29/ghc-0.29-src.tar.gz
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