[Haskell-cafe] more thoughts on "Finally tagless"

Tom Schrijvers tom.schrijvers at cs.kuleuven.be
Mon Mar 8 15:14:54 EST 2010

> Yeah, subject "Finally Tagless" again, sorry, I'm just not done with it yet.
> In Olegs haskell implementation he is using classes mainly to model the
> syntax and instances to use for evaluators / compilers to allow multiple
> interpretations.
> I wonder if it'd be possible to do the same without classes using data types
> instead?
> Something similar to what Luke Palmer has done here:
> http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-typeclass/


You can just use the dictionary translation of type classes to replace
them with data types:


class Eval sem where
  val :: Int -> sem Int
  add :: sem Int -> sem Int -> sem Int


data EvalDict sem = EvalDict { val :: Int -> sem Int, add :: sem Int
-> sem Int -> sem Int }



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