[Haskell-cafe] Reflective capabilities of Haskell

Martin Hofmann martin.hofmann at uni-bamberg.de
Tue Mar 11 06:57:52 EDT 2008

I am trying to port a programme written in Maude, which is a reflective
language based on rewriting logic ( http://maude.cs.uiuc.edu/ ), to
Haskell. I thought using Template Haskell might be a good idea, but I
got stuck and now I am wondering if this is really possible in Haskell.
Let me give an example of a Maude module defining the function last over
a list of Peano numbers. 

fmod LAST is
	sorts Peano PeanoList .
	op 0 : -> Peano [ctor] .
	op s : Peano -> Peano [ctor] .
	op [] : -> PeanoList [ctor] .
	op cons : Peano PeanoList -> PeanoList [ctor] .
	op last : PeanoList -> Peano .
	vars X Y : Peano .
	var Xs : PeanoList .
	eq last(cons(X,[])) = X .
	eq last(cons(X,cons(Y,Xs))) = last(cons(Y,Xs)) .

So, last(cons(s(0),cons(s(s(0)),cons(s(s(s(0))),[])))) would reduce to
s(s(s(0))). The cool thing about Maude is, that terms, modules, ... can
be lifted to the meta level. For example:

upModule('LAST, false)

would yield

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