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This is the root page for examples and issues about functional dependencies in GHC.

Main sub-pages

IMPORTANT This list of sub-pages have most of the content!

• Background and terminology
• Key examples
• Design choices

The rest this page gathers other useful links.

Key papers

• Type classes with functional dependencies, Mark Jones, ESOP 2000. The original fundep paper.
• Understanding functional dependencies via constraint handling rules, Sulzmann et al, JFP 2006. This journal paper has a lot of examples and we cite it frequently below as "JFP-paper".
• Elaboration on functional dependencies, Karachalias and Schrijvers, Haskell Symposium 2017. This paper shows how to translate functional dependencies into type families.
• Evidenced functional dependencies a wiki page developing the Karachalias and Schrijvers paper further.

Key proposals

• Coverage condition: Per-instance DYSFUNCTIONAL pragma.
• Instance consistency condition: Explore ways to weaken or abandon the Instance Consistency Condition.

GHC today

GHC today does this:

• By default: GHC uses the strict coverage condition and imposes the Paterson conditions.

• If UndecidableInstances is on, GHC

  • uses the liberal (not strict) coverage condition
  • lifts the Paterson conditions

• Always: GHC implements liberal instance consistency unconditionally. See Note [Bogus consistency check] in GHC.Tc.Instance.Fundeps. (GHC presumably does this because SICC is incompatible with LCC.)