Peter Verswyvelen wrote:
In Scheme, on can "quote" code, so that it becomes data. Microsoft's F# and C# 3.0 also have something similar that turns code into "expression trees".
I can't find something similar for Haskell? Maybe I am looking at the wrong places?
Quoting/Inspecting code at runtime is not possible in Haskell since this would break referential transparency, i.e. one could tell that two extensionally equal values like 3 `add` 4 1 `add` (2 `mul` 3) are different by inspecting their internal structure. Of course, you can use constructors for add and mul and then inspect and transform the result data Expr = Val Int | Add Expr Expr | Mul Expr Expr add, mul :: Expr -> Expr -> Expr add = Add mul = Mul x :: Expr x = Val 1 `add` (Val 2 `mul` Val 3) By making Expr an instance of the class Num , you can use overloaded arithmetic operations instance Num Expr where (+) = Add (*) = Mul fromInteger = Val . fromInteger x :: Expr x = 1 + 2*3
I want to write something like
selectiveQuote [add] (1 `add` 2 `mul` 3)
which would result in an expression tree like
add / \ 1 6
So the `mul` is not quoted because it is not part of the "context" = [add]
I'm not sure why you'd want to do that, but it's not well-defined. What would selectiveQuote [add] ((1 `add` 2) `mul` 3) be? How to expand `mul` here when `add` isn't expanded? Regards, apfelmus