Do Be Do Be Do

Sam Lindley, Conor McBride, Craig McLaughlin

Research output: Chapter in Book/Report/Conference proceedingConference contribution book

53 Citations (Scopus)
211 Downloads (Pure)


We explore the design and implementation of Frank, a strict functional programming language with a bidirectional effect type system designed from the ground up around a novel variant of Plotkin and Pretnar's effect handler abstraction.

Effect handlers provide an abstraction for modular effectful programming: a handler acts as an interpreter for a collection of commands whose interfaces are statically tracked by the type system. However, Frank eliminates the need for an additional effect handling construct by generalising the basic mechanism of functional abstraction itself. A function is simply the special case of a Frank operator that interprets no commands. Moreover, Frank's operators can be multihandlers which simultaneously interpret commands from several sources at once, without disturbing the direct style of functional programming with values.

Effect typing in Frank employs a novel form of effect polymorphism which avoid mentioning effect variables in source code. This is achieved by propagating an ambient ability inwards, rather than accumulating unions of potential effects outwards.

We introduce Frank by example, and then give a formal account of the Frank type system and its semantics. We introduce Core Frank by elaborating Frank operators into functions, case expressions, and unary handlers, and then give a sound small-step operational semantics for Core Frank.

Programming with effects and handlers is in its infancy. We contribute an exploration of future possibilities, particularly in combination with other forms of rich type system.
Original languageEnglish
Title of host publicationPOPL'2017
Subtitle of host publicationProceedings of the 44th ACM SIGPLAN Symposium on Principles of Programming Languages
EditorsAndrew Gordon
Place of PublicationNew York
Number of pages15
Publication statusPublished - 15 Jan 2017
EventThe 44th ACM SIGPLAN Symposium on Principles of Programming Languages: the 44th ACM SIGPLAN Symposium on Principles of Programming Languages - Paris, France
Duration: 15 Jan 201721 Jan 2017


ConferenceThe 44th ACM SIGPLAN Symposium on Principles of Programming Languages
Abbreviated titlePOPL 2017


  • algebraic effects
  • effect handlers
  • effect polymorphism
  • call-by-push-value
  • pattern matching
  • continuations
  • bidirectional typing


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