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I Got Plenty o’ Nuttin’

  • Conor McBrideEmail author
Part of the Lecture Notes in Computer Science book series (LNCS, volume 9600)

Abstract

Work to date on combining linear types and dependent types has deliberately and successfully avoided doing so. Entirely fit for their own purposes, such systems wisely insist that types depend only on the replicable sublanguage, thus sidestepping the issue of counting uses of limited-use data either within types or in ways which are only really needed to shut the typechecker up. As a result, the linear implication (‘lollipop’) stubbornly remains a non-dependent \(S \multimap T\). This paper defines and establishes the basic metatheory of a type theory supporting a ‘dependent lollipop’ \((x\!:\!S)\multimap T[x]\), where what the input used to be is in some way commemorated by the type of the output. For example, we might convert list to length-indexed vectors in place by a function with type \((l\!:\!\mathsf {List}\,X)\multimap \mathsf {Vector}\,X\,(\mathsf {length}\,l)\). Usage is tracked with resource annotations belonging to an arbitrary rig, or ‘riNg without Negation’. The key insight is to use the rig’s zero to mark information in contexts which is present for purposes of contemplation rather than consumption, like a meal we remember fondly but cannot eat twice. We need no runtime copies of l to form the above vector type. We can have plenty of nothing with no additional runtime resource, and nothing is plenty for the construction of dependent types.

Keywords

Function Type Type Theory Dependent Type Application Rule Type Annotation 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgements

Writing was a substitute for sleep in a busy summer of talks in the Netherlands, Germany and England. I thank everyone who interacted with me in the daytimes at least for their forbearance, and some for very useful conversations and feedback, notably James McKinna and Shayan Najd. The ideas were incubated during my visit to Microsoft Research Cambridge in 2014: I’m grateful to Nick Benton for feedback and encouragement. My talk on this topic at SREPLS in Cambridge provoked a very helpful interaction with Andy Pitts, Dominic Orchard, Tomas Petricek, and Stephen Dolan—his semiring-sensitivity provoked the generality of the resource treatment here [10]. I apologize to and thank the referees of all versions of this paper. Sam Lindley and Craig McLaughlin also deserve credit for helping me get my act together.

The inspiration, however, comes from Phil. The clarity of the connection he draws between classical linear logic and session types [31] is what attracted me to this problem. One day he, Simon Gay and I bashed rules at my whiteboard, trying to figure out a dependent version of that ‘Propositions As Sessions’ story. The need to distinguish ‘contemplation’ from ‘consumption’ emerged that afternoon: it has not yet delivered a full higher-order theory of ‘dependent session types’, but in my mind, it showed me the penny I had to make drop.

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Copyright information

© Springer International Publishing Switzerland 2016

Authors and Affiliations

  1. 1.Mathematically Structured ProgrammingUniversity of StrathclydeGlasgowScotland, UK

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