MultiParamTypeClasses-dilemma.md

+# Multi Parameter Type Classes Dilemma
+
+
+
+[MultiParamTypeClasses](multi-param-type-classes) are very useful, but mostly in the context of [FunctionalDependencies](functional-dependencies).  They are particularly used in the [ monad transformer library](http://www.haskell.org/ghc/docs/latest/html/libraries/index.html) found in fptools.  The dilemma is that [ functional dependencies are "very, very tricky"](http://www.haskell.org//pipermail/haskell-prime/2006-February/000289.html) (spj).  [AssociatedTypes](associated-types) are promising but unproven.  Without a solution, Haskell' will be somewhat obsolete before it gets off the ground.
+
+
+## References
+
+
+- [ Understanding functional dependencies via Constraint Handling Rules](http://research.microsoft.com/%7Esimonpj/papers/fd%2Dchr/), Martin Sulzmann, Gregory J. Duck, Simon Peyton Jones, and Peter J. Stuckey. Revised Feb 2006
+- P. J. Stuckey and M. Sulzmann. [ A theory of overloading](http://www.comp.nus.edu.sg/~sulzmann/chr/download/theory-journal.ps.gz). ACM Transactions on Programming Languages and Systems (TOPLAS), 27(6):1-54, 2005.
+
+## Options for solving the dilemma
+
+
+1. Examine a limited version of functional dependencies (see below for one solution)
+1. Put [AssociatedTypes](associated-types) on the fast-track for sainthood
+1. Punt: declare [FunctionalDependencies](functional-dependencies) as an extension and figure out a way to "bless" extensions
+1. Find a subset of [FunctionalDependencies](functional-dependencies) that is guarenteed to be a subset of what is expressible with [AssociatedTypes](associated-types). This would ensure no Haskell' programs become unexpressable if FDs switched to [AssociatedTypes](associated-types) in the future.
+
+## Options that don't actually solve the dilemma
+
+
+1. Standardize without [MultiParamTypeClasses](multi-param-type-classes) or [FunctionalDependencies](functional-dependencies)
+1. Standardize [FunctionalDependencies](functional-dependencies) even though [ they are tricky](http://www.haskell.org//pipermail/haskell-prime/2006-February/000289.html).
+
+## Tickets
+
+
+- [solve the MultiParamTypeClassDilemma](https://gitlab.haskell.org//haskell/prime/issues/90)
+
+## Proposed Solutions
+
+
+1.  Adopt functional dependencies with the FD-conditions from the [ CHR paper](http://research.microsoft.com/Users/simonpj/Papers/fd-chr/) together with the "complex relaxation rule" described at the bottom of the [FunctionalDependencies](functional-dependencies) page (also see the CHR paper, sect. 6).
+
+
+Pro: 
+
+
+- allows a safe version of [FunctionalDependencies](functional-dependencies) that can support the monad transformer library
+
+
+Con:
+
+
+- complex specification for what is and is not allowed in classes and instance with functional dependencies
+- not clear whether this could be forwards-compatible with associated types