Commit 38febb30 by dimitris@microsoft.com

### Commentary changes

parent daa3fb94
 ... ... @@ -292,6 +292,12 @@ isDerived _ = False canSolve :: CtFlavor -> CtFlavor -> Bool -- canSolve ctid1 ctid2 -- The constraint ctid1 can be used to solve ctid2 -- "to solve" means a reaction where the active parts of the two constraints match. -- active(F xis ~ xi) = F xis -- active(tv ~ xi) = tv -- active(D xis) = D xis -- active(IP nm ty) = nm ----------------------------------------- canSolve (Given {}) _ = True canSolve (Derived {}) (Wanted {}) = True canSolve (Derived {}) (Derived {}) = True ... ... @@ -300,14 +306,37 @@ canSolve _ _ = False canRewrite :: CtFlavor -> CtFlavor -> Bool -- canRewrite ctid1 ctid2 -- The *equality* constraint ctid1 can be used to rewrite inside ctid2 -- The *equality_constraint* ctid1 can be used to rewrite inside ctid2 canRewrite (Given {}) _ = True canRewrite (Derived {}) (Wanted {}) = True canRewrite (Derived {}) (Derived {}) = True -- Never use a wanted to rewrite anything! -- See note [Rewriting wanteds with wanteds] canRewrite (Wanted {}) (Wanted {}) = False canRewrite _ _ = False \end{code} Note [Rewriting wanteds with wanteds] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We will currently never use a wanted to rewrite any other constraint (function @canRewrite@). If a rewriting was possible at all, we simply wait until the wanted equality becomes given or derived and then use it. This way we avoid rewriting by eventually insoluble wanteds, such as in the following situation: w1 : a ~ Int w2 : F a ~ a w3 : F Int ~ G Bool where 'a' is a skolem variable. If we rewrite using w1 inside w2 we will get the perhaps mystifying error at the end that we could not solve (a ~ Int) and (G Bool ~ Int). But there is no point in rewriting with a ~ Int as long as it is still wanted. Notice that on the other hand we often do solve one wanted from another, (function @canSolve@) for instance in dictionary interactions, which is a reaction that enables sharing both in the solver and in the final evidence produced. \begin{code} combineCtLoc :: CtFlavor -> CtFlavor -> WantedLoc -- Precondition: At least one of them should be wanted combineCtLoc (Wanted loc) _ = loc ... ...
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