instance Read Dynamic ?
Mon, 25 Mar 2002 12:55:48 -0800
On Monday 25 March 2002 12:00 pm, Dean Herington wrote:
> The type `Dynamic` is an instance of `Show` but not of `Read`. Is there
> some reason `Dynamic` could not be made an instance of `Read`? Has
> anyone extended `Dynamic` in that way?
Here is a snippet of code from the GHC Dynamic library; you will notice that
Dynamic objects contain a TypeRep and a data object, but the data object is
ignored by show (as it must be, since the Obj could be anything, showable or
data Dynamic = Dynamic TypeRep Obj
instance Show Dynamic where
-- the instance just prints the type representation.
showsPrec _ (Dynamic t _) =
showString "<<" .
showsPrec 0 t .
It is impossible to make a matching Read instance, because you lose
information when you show a Dynamic object; there is no way a read function
can reconstruct the data object's value.
It probably wouldn't be too difficult to make a readable version of Dynamic,
but it would only be able to hold datatypes that were instances of Read and
Show. I'd be interested to see if anyone else has implemented anything like
this; it seems like it could be pretty useful.
- Brian Huffman