Commit 38febb30 authored by dimitris@microsoft.com's avatar dimitris@microsoft.com
Browse files

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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