Emulation of a simple subset of Forth in Haskell seems easy. The trick, continuation-passing style, has been known for a long time. The trick underlies `functional unparsing' by Olivier Danvy. http://www.brics.dk/RS/98/12/BRICS-RS-98-12.pdf (published in JFP in 1998). Chris Okasaki later extended the technique http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.121.1890 http://dl.acm.org/citation.cfm?id=581699 He also noted huge types and slow compilation times. But there is another way. It is far more complex, and far fast. It is used in HSXML, which handles polyvariadic functions with literally thousands of arguments (some of my web pages are long). The following complete code illustrates the idea. {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleInstances, FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} -- Simple Forth module SimpleForth where -- The simple way: Danvy's Functional unparsing begin k = k () n st x k = k ((x::Int),st) add (n1,(n2,st)) k = k (n1+n2,st) end (top,st) = top t1 = begin n 1 n 2 add end -- Uncomment the following to get a hint why t2 is too slow -- t1 = begin _h n 1 n 2 add end -- Uncomment the following only if you are prepared to wait {- t2 = begin n 1 n 2 n 3 n 4 n 5 n 6 n 7 n 8 n 9 add add add add add add add add end -} -- A more complex but faster way -- Start with a stack 'stack' and then continue class Forth stack r where build :: stack -> r data End = End instance (a ~ stack) => Forth stack (End -> a) where build stack _ = stack data Add = Add -- Start with (Int, (Int, stack)), see Add and continue with (Int,stack) instance Forth (Int,stack) r => Forth (Int,(Int,stack)) (Add -> r) where build (n1,(n2,stack)) _ = build (n1+n2,stack) data N = N instance (a ~ Int, Forth (Int,stack) r) => Forth stack (N -> a -> r) where build stack _ n = build (n,stack) -- All of the following typecheck instantaneously, even on my slow -- laptop tt1 = build () N 1 N 2 Add End tt2 = build () N 1 N 2 N 3 N 4 N 5 N 6 N 7 N 8 N 9 Add Add Add Add Add Add Add Add End tt3 = build () N 1 N 2 Add N 3 Add N 4 Add N 5 Add N 6 Add N 7 Add N 8 Add N 9 Add End