Re: [Haskell-cafe] Typed DSL compiler, or converting from an existential to a concrete type

oleg@pobox.com wrote:

we might attempt to write

...

testr = either (error) (ev) (typecheck env0 te3) where ev (TypedTerm t e) = sin (eval e)

We know that it should work.

If we know it has to be a Double, we can express that:

testr = either (error) (ev) (typecheck env0 te3) where ev (TypedTerm TDouble e) = sin (eval e)

and this compiles. Of course, we can consider the other types as well:

testr = either (error) (ev) (typecheck env0 te3) where ev (TypedTerm TDouble e) = Right $ sin (eval e) ev _ = Left "not a Double"

Zun.