[Haskell-cafe] using Typeable with STRefs

Antoine Latter aslatter at gmail.com
Mon Mar 16 22:02:18 EDT 2009

On Mon, Mar 16, 2009 at 8:08 PM, Michael Vanier <mvanier42 at gmail.com> wrote:
> Ryan,
> So, if I understand you correctly, my only option is to use an IORef instead
> of an STRef?  What I'm trying to do is implement a mutable box type as part
> of a dynamically-typed language I'm implementing in Haskell (which is mainly
> an exercise to improve my Haskell programming; mission accomplished).  It
> bothers me that I have to use an IORef for this, since I don't see what this
> has to do with I/O.  Similarly, if I wanted to have a mutable array type, I
> couldn't use STArray; I'd have to use IOArray.  Or, I suppose I could define
> a richer Value type that had extra constructors for stateful types.

Why not:

data Value = forall a . Typeable a => V a

type STValue s = STRef Value

Then you have a dynamic, mutable box.

Also, even if the language you're interpretting is dynamicaly typed,
it doesn't mean that you need to use Haskell Dynamics.  It should be
enough to do something like:

data Value
 = VInt Integer
 | VStr String
 | VChar Char

and then:

-- | Similar to Prelude.Head
primOpHead :: Value -> Value
primOpHead (VStr xs) = VChar $ head xs
primOpHead _ = error "Type Mismatch!"

I'm sure other people can point you to much better ways to do this -
the point is that you don't need Dynamics to implement a dynamic


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