Changes between Version 96 and Version 97 of TypeFunctionsSolving

Timestamp:

09/29/08 22:05:22 (5 years ago)

Author:

chak

Comment:

--

TypeFunctionsSolving

@@ -250 +250 @@
  1.  '''Substitution:''' For any variable equality of the form `co :: x ~ t` (both local and wanted), we apply the substitution `[t/x]` to the '''left-hand side''' of all equalities.  We also perform the same substitution on all class constraints.
  2. '''Instantiation:''' For any variable equality of the form `co :: alpha ~ t` or `co :: a ~ alpha`, where `co` is wanted, we instantiate `alpha` with `t` or `a`, respectively, and set `co := id`.  Moreover, we have to do the same for equalities of the form `co :: F t1..tn ~ alpha` unless we are in inference mode and `alpha` appears in the environment or any other wanteds. (We never instantiate any flexibles introduced by flattening locals.)
-The substitution step can lead to recursive equalities; i.e., we need to apply an occurs check after each substitution.  It is an important property of propagation that we only need to substitute into right-hand sides during finalisation.
-
-After finalisation and zonking all flattening of locals is undone (c.f., note below the flattening code above).
+The substitution step can lead to recursive equalities; i.e., we need to apply an occurs check after each substitution.  We need to instantiate all flexibles that arose during flattening of wanteds ''before'' we instantiate any other flexibles.  Consider `F delta ~ alpha, F alpha ~ delta`, where `alpha` is a skolem and `delta` a free flexible.  We need to produce `F (F delta) ~ delta` (and not `F (F alpha) ~ alpha`).  Otherwise, we may wrongly claim to having performed an improvement, which can lead to non-termination of the combined class-family solver.
+
+Note that it is an important property of propagation that we only need to substitute into right-hand sides during finalisation.  After finalisation and zonking all flattening of locals is undone (c.f., note below the flattening code above).
 
 == Examples ==