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

Martijn van Steenbergen wrote:

Tom Schrijvers wrote:

...

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

An alternative option is to capture the structure in a GADT:

...

data Eval a where Val :: Int -> Eval Int Add :: Eval Int -> Eval Int -> Eval Int

And then write what were instances before as functions Eval a -> whatever.

But these interpreter functions Eval a -> whatever would have to use pattern matching at runtime to discover the structure of the term, so this approach is no longer tagless. Tillmann