dynamic types

[Hal Daume III]

You are correct.  You can't say:

  let x = if ??? then () else (5::Integer)

because then the compiler can't assign a type to x.

However, to achieve what you want, you might try something like this
(untested code follows):

> main = do
>    l <- getLine
>    let x = case maybeRead l of
>              Just () -> Left ()
>              _ -> case maybeRead l of
>                     Just (i::Integer) -> Right i
>    if isLeft x
>      then putStrLn "Unit type detected"
>      else putStrLn "Integer type detected"

where

> maybeRead :: Read a => String -> Maybe a
> maybeRead x = case readsPrec x of
>                 [(a,_)] -> Just a
>                 _ -> Nothing

or something like that.

Another way to do it would be to define a "universal" type:

> data Univ = UUnit () | UInteger Integer | UDouble Double | ...

and define an instance of read on it that loos like the cascaded cases
from above...

HTH,

  Hal

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

On Tue, 7 Jan 2003, Richard Uhtenwoldt wrote:

> Just want to make sure I understand.
> 
> There does not exist a Haskell implementation-specific extension which
> provides the sort of dynamic type that would allow one to discriminate
> at run time between built-in types; right?  eg,
> 
> main=do 
>     object<-readLn
>     case object of   --or some such syntax
>         ()->putStrLn "void type detected"
>         (a::Integer)->putStrLn "Integer type detected"
> 
> For those readers whose prelude is rusty, I quote from the prelude:
> 
>    readLn           :: Read a => IO a
>    readLn           =  do l <- getLine
>                           r <- readIO l
>                           return r
> 
> 
> And even if the object being discriminated "lacks
> polymorphicity", eg, this next, that does not change the answer
> to my question; right?
> 
> main=do 
>     command<-readLn
>     let 
>         object=case command of
>             "Integer"->(5::Integer)
>             "Void"->()
>     case object of
>         ()->putStrLn "void type detected"
>         (a::Integer)->putStrLn "Integer type detected"
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