[Haskell-cafe] Programming an Evaluation Context

[Claus Reinke]

> -- evaluation contexts, parameterized by rule to apply
> ctxt rule e = rule e `mplus` left (ctxt rule) e `mplus` right (ctxt rule) e
> 
> left  act e = do { (Plus l r)          <- return e; lv <- act l; return (Plus lv r) }
> right act e = do { (Plus lv@(Val _) r) <- return e; rv <- act r; return (Plus lv rv) }

actually, for the common case of reduction in context (contexts being preserved
unchanged), we can build contexts from context constructors corresponding to the 
data constructors. moving the common plumbing into the context constructors 
makes the context specification itself a lot clearer (a transliteration of the formal
specification:-):

    -- context constructors
    val  fi    e = do { Val i    <- return e; return (Val (fi i)) }
    plus cl cr e = do { Plus l r <- return e; l' <- cl l; r' <- cr r; return (Plus l' r') }

    expr      e = return e
    (a .|. b) e = a e `mplus` b e

    -- evaluation contexts, parameterized by rule to apply, abstract style
    ctxt' hole = hole .|. plus (ctxt' hole) expr .|. plus (val id) (ctxt' hole) 

Claus