Changes between Version 98 and Version 99 of TypeFunctionsSolving
 Timestamp:
 Apr 5, 2009 10:35:32 AM (6 years ago)
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TypeFunctionsSolving
v98 v99 248 248 249 249 The finalisation step instantiates as many flexible type variables as possible, but it takes care not to instantiate variables occurring in the global environment with types containing synonym family applications. This is important to obtain principle types (c.f., Andrew Kennedy's thesis). We perform finalisation in two phases: 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 allclass constraints.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 '''righthand side''' of all equalities. We also perform the same substitution on 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.) 252 252 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.