[Haskell-cafe] Introspection

[Sharadha Raghunathan]

But here we need to get the grammar from users. How can I do that? Or how can input the grammar in my code?? 

Sent from my iPhone

On Jul 22, 2011, at 5:04 AM, Yves Parès <limestrael at gmail.com> wrote:

> You can do that using the Typeable class and possibly the Dynamic type.
> (Data.Typeable and Data.Dynamic in base)
> 
> When you have:
> 
> foo :: (Typeable a, Num a) => a -> a
> foo x = case (cast x :: Maybe Int) of
>     Just x' -> x + 12
>     _ -> x
> 
> foo will accept every instance of Num but will add 12 to it only if it is an Int.
> 
> Dynamic wraps an instance of Typeable.
> 
> 2011/7/22 Patrick Browne <patrick.browne at dit.ie>
> Is it possible to access type information and perform either of the
> following in a Haskell program?
> 
> 1) Find the type/class of a variable
> e.g. a type predicate:  is y of-type A, or is y of-Class A
> 
> 2) Assert the type of a variable
> e.g. if y.length > 100 then y is of type big.
> 
> Regards,
> Pat
> 
> 
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