Changes between Version 97 and Version 98 of TypeFunctionsSolving
 Timestamp:
 Sep 30, 2008 5:09:45 AM (8 years ago)
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TypeFunctionsSolving
v97 v98 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 '''lefthand side''' of all equalities. We also perform the same substitution on all class constraints. 251 251 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.) 252 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 nontermination of the combined classfamily solver.252 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 as skolems 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 nontermination of the combined classfamily solver. 253 253 254 254 Note that it is an important property of propagation that we only need to substitute into righthand sides during finalisation. After finalisation and zonking all flattening of locals is undone (c.f., note below the flattening code above).