Evaluation strategies for monadic computations

Tomas Petricek

In Proceedings of MSFP 2012

Monads have become a powerful tool for structuring effectful computations in functional programming, because they make the order of effects explicit. When translating pure code to a monadic version, we need to specify evaluation order explicitly. Two standard translations give call-by-value and call-by-name semantics. The resulting programs have different structure and types, which makes revisiting the choice difficult.

In this paper, we translate pure code to monadic using an additional operation malias that abstracts out the evaluation strategy. The malias operation is based on computational comonads; we use a categorical framework to specify the laws that are required to hold about the operation.

For any monad, we show implementations of malias that give call-by-value and call-by-name semantics. Although we do not give call-by-need semantics for all monads, we show how to turn certain monads into an extended monad with call-by-need semantics, which partly answers an open question. Moreover, using our unified translation, it is possible to change the evaluation strategy of functional code translated to the monadic form without changing its structure or types.

Paper and more information

• Download the paper pre-print (PDF)
• Download the published paper (PDF), from arXiv
• View slides from MSFP 2012 (PDF)
• Download associated Haskell source code (ZIP)

Related papers

The malias abstraction has been partly inspired by the author's work on joinads which add pattern matching support to monadic computations. Joinads require the malias operation for a related reason - for more information see also Extending monads with pattern matching.

Bibtex

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@inproceedings{malias-msfp12,
  author    = {Petricek, Tomas},
  title     = {Evaluations strategies for monadic computations},
  booktitle = {Proceedings of Mathematically Structured Functional Programming},
  series    = {MSFP 2012},
  location  = {Tallinn, Estonia},
  year      = {2012}
}

If you have any comments, suggestions or related ideas, I'll be happy to hear from you! Send me an email at tomas@tomasp.net or get in touch via Twitter at @tomaspetricek.