[Haskell-cafe] Difficulties with tagless - create "primitives" or compose them

[Günther Schmidt]

Hi list,

there is one thing about the "Finally tagless" EDSL approach that really 
confuses me: (Well more than one actually but this one more so)

When to decide to introduce a term as a primitive in the syntax class 
and when to define it from primitives already defined.

For example this one here:

type Arr exp a b = exp a -> exp b

class EDSL exp where
      lam :: (exp a -> exp b) -> exp (Arr exp a b)
      app :: exp (Arr exp a b) -> exp a -> exp b

      int :: Int -> exp Int		-- Integer literal
      add :: exp Int -> exp Int -> exp Int
      sub :: exp Int -> exp Int -> exp Int
      mul :: exp Int -> exp Int -> exp Int

Let's take "mul" here, defined as a "primitive", in other words defined 
in the EDSL class.

Technically, with lam, app and add already defined, I could have defined 
"mul" outside the EDSL class, just built from the 3 primitive operators.

Of course doing so then does not give me the possibility to choose 
alternative evaluation strategies for "mul" itself, only for lam, app 
and add.

So what is a good measure for deciding when to define a term primitive 
or not?

Günther