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Comprehending Ringads: For Phil Wadler‚ on the Occasion of his 60th Birthday

Jeremy Gibbons

Abstract

List comprehensions are a widely used programming construct, in languages such as Haskell and Python and in technologies such as Microsoft's Language Integrated Query. They generalize from lists to arbitrary monads, yielding a lightweight idiom of imperative programming in a pure functional language. When the monad has the additional structure of a so-called ringad, corresponding to "empty" and "union" operations, then it can be seen as some kind of collection type, and the comprehension notation can also be extended to incorporate aggregations. Ringad comprehensions represent a convenient notation for expressing database queries. The ringad structure alone does not provide a good explanation or an efficient implementation of relational joins; but by allowing heterogeneous comprehensions, involving both bag and indexed table ringads, we show how to accommodate these too.

Book Title
A List of Successes That Can Change the World
Month
April
Pages
132−151
Publisher
Springer
Series
LNCS
Volume
9600
Year
2016