Changes between Version 23 and Version 24 of FunctionalDependencies

Timestamp:

05/23/06 06:03:00 (7 years ago)

Author:

ross@…

Comment:

expand use cases

FunctionalDependencies

@@ -24 +24 @@
 [[TicketQuery(description~=FunctionalDependencies)]]
 
-== Use Cases ==
- * Edison data structures library
- * Monad Transformers library
- * possible use in Hat tracer [attachment:thoughtsOnMixing.ps]
-
 == Pros ==
  * In GHC and Hugs for a long time.
@@ -100 +95 @@
  * With FlexibleInstances and no OverlappingInstances, this system is coherent and decidable (CHR paper, corr. 1).
 
- * Unfortunately the "coverage" condition rules out instances like the following, from the monad transformer library:
-   {{{
-class (Monoid w, Monad m) => MonadWriter w m | m -> w
-instance (Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)
-}}}
+ * Because it ignores the context, the "coverage" condition does not allow some instances for "transformere", e.g. in the monad transformer library (see [wiki:FunctionalDependencies#UseCases Use Cases] below).
 
 == GHC and Hugs ==
@@ -188 +179 @@
 Another possibility is to modify instance reduction to record the functional dependency on the constraint being reduced (see [http://www.cs.kent.ac.uk/~cr3/chr/Tc2CHR.hs]).
 
+== Use Cases ==
+
+ [http://www.haskell.org/ghc/docs/6.4/html/libraries/mtl/ Monad Transformers library] ([http://www.csee.ogi.edu/~diatchki/monadLib new version])::
+   Auxiliary parameters (state, errors, output, environment) are determined by the monad, e.g.:
+   {{{
+class (Monoid w, Monad m) => MonadWriter w m | m -> w
+}}}
+   All dependencies are full, and there are two sorts of instances:
+   base case satisfying the coverage condition, e.g.
+   {{{
+instance (Monoid w, Monad m) => MonadWriter w (WriterT w m)
+}}}
+   and transformer applications where the dependency is derived from dependencies in the context, e.g.
+   {{{
+instance (Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)
+}}}
+
+ [http://www.eecs.tufts.edu/~rdocki01/edison.html Edison] data structures library::
+   Keys (or elements of sets) are determined by the collection, e.g.
+   {{{
+class (Coll c a, OrdCollX c a) => OrdColl c a | c -> a
+}}}
+   The most complex case is represented by adaptors like
+   {{{
+instance (OrdColl h a, Ord a) => OrdColl (Min h a) a
+}}}
+   All dependencies are full and all instances satisfy the coverage condition.
+
+ [http://darcs.haskell.org/packages/collections/ collections] package::
+   Elements are determined by the collection, e.g.
+   {{{
+class Foldable c o => Collection c i o | c -> i o
+}}}
+   The most complex case is represented by views like
+   {{{
+instance Collection m (k,v) (k,v) => Collection (KeysView m k v) (k,v) k
+}}}
+   All dependencies are full and all instances satisfy the coverage condition.
+
+ [http://www.haskell.org/yampa/ Yampa]::
+   Vector spaces, with the scalar type determined by the vector type.
+   Some non-full dependencies, but all instances satisfy the coverage condition.
+
+ possible use in Hat tracer::
+   [attachment:thoughtsOnMixing.ps]
+
+Functional dependencies are often used in fairly experimental work together with UndecidableInstances and OverlappingInstances, but since those are unlikely to be included in Haskell', they are not considered further here.
+
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