[Haskell-cafe] Re: Is using Data.Dynamic considered a no-go?

Achim Schneider barsoap at web.de
Thu Feb 12 12:58:18 EST 2009

Peter Verswyvelen <bugfact at gmail.com> wrote:

> Haskell seems to have pretty strong support for dynamic casting using
> Data.Typeable and Data.Dynamic.
> All kinds of funky dynamic programming seems to be possible with these
> "hacks".
> Is this considered as being as bad as - say - unsafePerformIO? What
> kind of evil is lurking here?
Uncertainty. Noone on #haskell seems to know how Typeable-dispatches
compare to pattern matching wrt. performance, and I was either too lazy
or too uninterested to measure it myself. Combined with a custom type
class and existentials, they safe you from writing catch-all ADTs and
thus give code flexibility (and conciseness), while retaining static
typing, modulo unexhaustive pattern warnings. This is how XHB uses
Typeable, and, IMHO, it's a Good Thing.

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