Re: [Haskell-cafe] Newbie question on Haskell type

14 Oct 2005

      So this is essentially a parsing problem.  You want a user to be able 
input a string and have it interpreted as an appropriate data value.  I 
think you may want to look at the Parsec library 
(http://www.cs.uu.nl/~daan/parsec.html).  I don't think the direction 
you are heading will get the results you want.

As to typeable, the basic types are mostly all members of Typeable.  You 
can find a pretty good list here:

http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data.Typeable.htm...

Additionally, GHC can create Typeable instances automaticly for user 
defined datatypes; just add a deriving Typeable clause.

data SomeType = C1 | C2 deriving (Show,Eq,Typeable)

The restriction is that all types which appear in constructors must also 
be in Typeable.

I believe the DrIFT preprocessor can also create Typeable instances if 
you are not using GHC.

Huong Nguyen wrote:
...
Hello,
Thanks for your solution.
My main purpose is that I want to input a value and check whether this 
value is belong to some specific types or not. These types can be some 
popular types (such as: String, Char, Int, etc) or some more complex 
data structures defined by user. Thus, at first, I try with type String 
(even with that simple type, I still face difficulty ;-))
I want to ask you which types can be used with Data.Typeable. I read for 
over 15 minutes but it is still not clear with me.
For some other complex data types defined by user, what I should do to 
use Data.Typeable ?
Thank you very much.
On 10/13/05, *robert dockins* mailto:robdockins@fastmail.fm> wrote:
In GHC you can do this:
> import Data.Typeable
> isString :: (Typeable a) => a -> Bool
     > isString x = typeOf x == typeOf (undefined::String)
Why do you want this?  It's not the kind of operation one does very
    often in Haskell.
Huong Nguyen wrote:
> Hi all,
     >
     > I want to write a small functionto test whether an input is a
    String or
     > not. For example,
     >
     > isString::(Show a) =>a ->Bool
     > This function will return True if the input is a string and
    return False
     > if not
     >
     > Any of you have idea about that? Thanks in advance
     >
     >
     >
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