Commit c5977d4d authored by Sebastian Graf's avatar Sebastian Graf Committed by Marge Bot
Browse files

Better documentation for mkEtaWW [skip ci]

So that hopefully I understand it faster next time. Also got rid of the
confusing `orig_expr`, which makes the call site in `etaExpand` look out
of sync with the passed `n` (which is not the original `n`).
parent 30be3bf1
......@@ -938,7 +938,7 @@ etaExpand :: Arity -- ^ Result should have this number of value arg
etaExpand n orig_expr
= go n orig_expr
-- Strip off existing lambdas and casts
-- Strip off existing lambdas and casts before handing off to mkEtaWW
-- Note [Eta expansion and SCCs]
go 0 expr = expr
go n (Lam v body) | isTyVar v = Lam v (go n body)
......@@ -949,7 +949,7 @@ etaExpand n orig_expr
retick $ etaInfoAbs etas (etaInfoApp subst' sexpr etas)
in_scope = mkInScopeSet (exprFreeVars expr)
(in_scope', etas) = mkEtaWW n orig_expr in_scope (exprType expr)
(in_scope', etas) = mkEtaWW n (ppr orig_expr) in_scope (exprType expr)
subst' = mkEmptySubst in_scope'
-- Find ticks behind type apps.
......@@ -1040,14 +1040,27 @@ etaInfoAppTy ty (EtaVar v : eis) = etaInfoAppTy (applyTypeToArg ty (varToCoreExp
etaInfoAppTy _ (EtaCo co : eis) = etaInfoAppTy (coercionRKind co) eis
mkEtaWW :: Arity -> CoreExpr -> InScopeSet -> Type
-> (InScopeSet, [EtaInfo])
-- EtaInfo contains fresh variables,
-- not free in the incoming CoreExpr
-- Outgoing InScopeSet includes the EtaInfo vars
-- and the original free vars
mkEtaWW orig_n orig_expr in_scope orig_ty
-- | @mkEtaWW n _ fvs ty@ will compute the 'EtaInfo' necessary for eta-expanding
-- an expression @e :: ty@ to take @n@ value arguments, where @fvs@ are the
-- free variables of @e@.
-- Note that this function is entirely unconcerned about cost centres and other
-- semantically-irrelevant source annotations, so call sites must take care to
-- preserve that info. See Note [Eta expansion and SCCs].
:: Arity
-- ^ How many value arguments to eta-expand
-> SDoc
-- ^ The pretty-printed original expression, for warnings.
-> InScopeSet
-- ^ A super-set of the free vars of the expression to eta-expand.
-> Type
-> (InScopeSet, [EtaInfo])
-- ^ The variables in 'EtaInfo' are fresh wrt. to the incoming 'InScopeSet'.
-- The outgoing 'InScopeSet' extends the incoming 'InScopeSet' with the
-- fresh variables in 'EtaInfo'.
mkEtaWW orig_n ppr_orig_expr in_scope orig_ty
= go orig_n empty_subst orig_ty []
empty_subst = mkEmptyTCvSubst in_scope
......@@ -1104,7 +1117,7 @@ mkEtaWW orig_n orig_expr in_scope orig_ty
| otherwise -- We have an expression of arity > 0,
-- but its type isn't a function, or a binder
-- is levity-polymorphic
= WARN( True, (ppr orig_n <+> ppr orig_ty) $$ ppr orig_expr )
= WARN( True, (ppr orig_n <+> ppr orig_ty) $$ ppr_orig_expr )
(getTCvInScope subst, reverse eis)
-- This *can* legitimately happen:
-- e.g. coerce Int (\x. x) Essentially the programmer is
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