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Tail recursion modulo cons

Tail recursion modulo cons is a generalisation of tail recursion introduced by D.H.D. Warren. It applies when the last thing a function does is to apply a constructor functions (e.g. cons) to an application of a non-primitive function. This is transformed into a tail call to the function which is also passed a pointer to where its result should be written. For example,
	f []     = []
	f (x:xs) = 1 : f xs
is transformed into (pseudo C/Haskell):
	f [] = []
	f l  = f' l allocate_cons

f' [] p = { *p = nil; return *p } f' (x:xs) p = { cell = allocate_cons; *p = cell; cell.head = 1; return f' xs &cell.tail }

where allocate_cons returns the address of a new cons cell, *p is the location pointed to by p and &c is the address of c.

[D.H.D. Warren, DAI Research Report 141, University of Edinburgh 1980].


This article was originally based on material from FOLDOC, used with permission. Update as needed.