Changes between Version 94 and Version 95 of TypeFunctionsSolving


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Timestamp:
Sep 17, 2008 7:03:55 AM (6 years ago)
Author:
chak
Comment:

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  • TypeFunctionsSolving

    v94 v95  
    245245The 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: 
    246246 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. 
    247  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`.  (We never instantiate any flexibles introduced by flattening locals.) 
     247 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.) 
    248248The 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. 
    249249