Headline

A program that manipulates programs

Illustration

Consider the following simplify for arithmetic expressions, written in Language:Haskell:

-- Simple arithmetic expressions
data Expr = Const Int | Add Expr Expr
  deriving (Eq, Show)

-- Simplification for unit law of addition
simplify :: Expr -> Expr
simplify t@(Const c) = t
simplify (Add (Const 0) x) = simplify x
simplify (Add x (Const 0)) = simplify x
simplify t@(Add x y) = 
   if t==t'
     then t
     else simplify t'
  where
    t' = Add (simplify x) (simplify y)

For instance, the simplifier works as follows:

> simplify (Add (Const 20) (Const 22))
Add (Const 20) (Const 22)
> simplify (Add (Const 0) (Const 42))
Const 42

The simplifier is a metaprogram, as it is a program that manipulates another program. The metalanguage used here is Language:Haskell. The object language used here is Expr, the language of expression.

See the concept of reflection for a more specific form of metaprogramming for further illustrations.

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