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Arrows, like monads, are monoids
 Proc. of 22nd Ann. Conf. on Mathematical Foundations of Programming Semantics, MFPS XXII, v. 158 of Electron. Notes in Theoret. Comput. Sci
, 2006
"... Monads are by now wellestablished as programming construct in functional languages. Recently, the notion of “Arrow ” was introduced by Hughes as an extension, not with one, but with two type parameters. At first, these Arrows may look somewhat arbitrary. Here we show that they are categorically fai ..."
Abstract

Cited by 12 (1 self)
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Monads are by now wellestablished as programming construct in functional languages. Recently, the notion of “Arrow ” was introduced by Hughes as an extension, not with one, but with two type parameters. At first, these Arrows may look somewhat arbitrary. Here we show that they are categorically fairly civilised, by showing that they correspond to monoids in suitable subcategories of bifunctors C op ×C → C. This shows that, at a suitable level of abstraction, arrows are like monads — which are monoids in categories of functors C → C. Freyd categories have been introduced by Power and Robinson to model computational effects, well before Hughes ’ Arrows appeared. It is often claimed (informally) that Arrows are simply Freyd categories. We shall make this claim precise by showing how monoids in categories of bifunctors exactly correspond to Freyd categories.
Functorial ML
, 1998
"... We present an extension of the HindleyMilner type system that supports a generous class of type constructors called functors, and provide a parametrically polymorphic algorithm for their mapping, i.e. for applying a function to each datum appearing in a value of constructed type. The algorithm come ..."
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We present an extension of the HindleyMilner type system that supports a generous class of type constructors called functors, and provide a parametrically polymorphic algorithm for their mapping, i.e. for applying a function to each datum appearing in a value of constructed type. The algorithm comes from shape theory, which provides a uniform method for locating data within a shape. The resulting system is ChurchRosser and strongly normalizing, and supports type inference. Several dierent semantics are possible, which aects the choice of constants in the language, and are used to illustrate the relationship to polytypic programming. Capsule Review A wide class of type constructors (functions producing types from types) used in functional programming are functorial, in the sense that they can be extended to mappings from functions to functions satisfying a few simple laws. The `map' functional for lists is the prototypic example. Moreover, this additional structure for type constru...