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INFORMATICA, 1999, Vol. 10, No. 1, 5-26
© Institute of Mathematics and Informatics, Vilnius, 1998

ISSN 0868-4952

Primitive (Co)Recursion and Course-of-Value (Co)Iteration, Categorically

Tarmo UUSTALUa, Varmo VENEb

aDept. of Teleinformatics, Royal Inst.of Technology, Electrum 204, SE-164 40 Kista, Sweden E-mail: tarmo@it.kth.se

bInst.of Computer Science, Univ.of Tartu, J. Liivi 2, EE-2484 Tartu, Estonia E-mail: varmo@cs.ut.ee

Abstract

In the mainstream categorical approach to typed (total) functional programming, datatypes are modelled as initial algebras and codatatypes as terminal coalgebras. The basic function definition schemes of iteration and coiteration are modelled by constructions known as catamorphisms and anamorphisms. Primitive recursion has been captured by a construction called paramorphisms. We draw attention to the dual construction of apomorphisms, and show on examples that primitive corecursion is a useful function definition scheme. We also put forward and study two novel constructions, viz., histomorphisms and futumorphisms, that capture the powerful schemes of course-of-value iteration and its dual, respectively, and argue that even these are helpful.

Keywords:

typed (total) functional programming, category theory, program calculation, (co)data-\break types, forms of (co)recursion

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