[Git][ghc/ghc][wip/T22719] Document the semantics of pattern bindings a bit better
Simon Peyton Jones (@simonpj)
gitlab at gitlab.haskell.org
Mon Jan 16 14:54:08 UTC 2023
Simon Peyton Jones pushed to branch wip/T22719 at Glasgow Haskell Compiler / GHC
Commits:
6752c1bf by Simon Peyton Jones at 2023-01-16T14:54:21+00:00
Document the semantics of pattern bindings a bit better
This MR is in response to the discussion on #22719
- - - - -
7 changed files:
- compiler/GHC/Hs/Utils.hs
- compiler/GHC/HsToCore/Expr.hs
- docs/users_guide/exts/primitives.rst
- docs/users_guide/exts/strict.rst
- + testsuite/tests/deSugar/should_compile/T22719.hs
- + testsuite/tests/deSugar/should_compile/T22719.stderr
- testsuite/tests/deSugar/should_compile/all.T
Changes:
=====================================
compiler/GHC/Hs/Utils.hs
=====================================
@@ -86,7 +86,7 @@ module GHC.Hs.Utils(
mkLetStmt,
-- * Collecting binders
- isUnliftedHsBind, isBangedHsBind,
+ isUnliftedHsBind, isUnliftedHsBinds, isBangedHsBind,
collectLocalBinders, collectHsValBinders, collectHsBindListBinders,
collectHsIdBinders,
@@ -905,55 +905,106 @@ to return a [Name] or [Id]. Before renaming the record punning
and wild-card mechanism makes it hard to know what is bound.
So these functions should not be applied to (HsSyn RdrName)
-Note [Unlifted id check in isUnliftedHsBind]
+Note [isUnliftedHsBind]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The function isUnliftedHsBind is used to complain if we make a top-level
-binding for a variable of unlifted type.
+The function isUnliftedHsBind tells if the binding binds a variable of
+unlifted type. e.g.
-Such a binding is illegal if the top-level binding would be unlifted;
-but also if the local letrec generated by desugaring AbsBinds would be.
-E.g.
- f :: Num a => (# a, a #)
- g :: Num a => a -> a
- f = ...g...
- g = ...g...
+ - I# x = blah
+ - Just (I# x) = blah
-The top-level bindings for f,g are not unlifted (because of the Num a =>),
-but the local, recursive, monomorphic bindings are:
+isUnliftedHsBind is used in two ways:
+* To complain if we make a top-level binding for a variable of unlifted
+ type. E.g. any of the above bindings are illegal at top level
+
+* To generate a case expression for a non-recursive local let. E.g.
+ let Just (I# x) = blah in body
+ ==>
+ case blah of Just (I# x) -> body
+ See GHC.HsToCore.Expr.dsUnliftedBind.
+
+Wrinkles:
+
+(W1) For AbsBinds we must check if the local letrec generated by desugaring
+ AbsBinds would be unlifted; so we just recurse into the abs_binds. E.g.
+ f :: Num a => (# a, a #)
+ g :: Num a => a -> a
+ f = ...g...
+ g = ...g...
+
+ The top-level bindings for f,g are not unlifted (because of the Num a =>),
+ but the local, recursive, monomorphic bindings are:
t = /\a \(d:Num a).
letrec fm :: (# a, a #) = ...g...
gm :: a -> a = ...f...
in (fm, gm)
-Here the binding for 'fm' is illegal. So generally we check the abe_mono types.
+ Here the binding for 'fm' is illegal. So we recurse into the abs_binds
+
+(W2) BUT we have a special case when abs_sig is true;
+ see Note [The abs_sig field of AbsBinds] in GHC.Hs.Binds
+
+(W3) isUnliftedHsBind returns False even if the binding itself is
+ unlifted, provided it binds only lifted variables. E.g.
+ - (# a,b #) = (# reverse xs, xs #)
+
+ - x = sqrt# y# :: Float#
+
+ - type Unl :: UnliftedType
+ data Unl = MkUnl Int
+ MkUnl z = blah
-BUT we have a special case when abs_sig is true;
- see Note [The abs_sig field of AbsBinds] in GHC.Hs.Binds
+ In each case the RHS of the "=" has unlifted type, but isUnliftedHsBind
+ returns False. Reason: see GHC Proposal #35
+ https://github.com/ghc-proposals/ghc-proposals/blob/master/
+ proposals/0035-unbanged-strict-patterns.rst
+
+(W4) In particular, (W3) applies to a pattern that binds no variables at all.
+ So { _ = sqrt# y :: Float# } returns False from isUnliftedHsBind, but
+ { x = sqrt# y :: Float# } returns True.
+ This is arguably a bit confusing (see #22719)
-}
----------------- Bindings --------------------------
-- | Should we treat this as an unlifted bind? This will be true for any
-- bind that binds an unlifted variable, but we must be careful around
--- AbsBinds. See Note [Unlifted id check in isUnliftedHsBind]. For usage
+-- AbsBinds. See Note [isUnliftedHsBind]. For usage
-- information, see Note [Strict binds checks] is GHC.HsToCore.Binds.
isUnliftedHsBind :: HsBind GhcTc -> Bool -- works only over typechecked binds
-isUnliftedHsBind bind
- | XHsBindsLR (AbsBinds { abs_exports = exports, abs_sig = has_sig }) <- bind
- = if has_sig
- then any (is_unlifted_id . abe_poly) exports
- else any (is_unlifted_id . abe_mono) exports
+isUnliftedHsBind (XHsBindsLR (AbsBinds { abs_exports = exports
+ , abs_sig = has_sig
+ , abs_binds = binds }))
+ | has_sig = any (is_unlifted_id . abe_poly) exports
+ | otherwise = isUnliftedHsBinds binds
+ -- See wrinkle (W1) and (W2) in Note [isUnliftedHsBind]
-- If has_sig is True we will never generate a binding for abe_mono,
-- so we don't need to worry about it being unlifted. The abe_poly
-- binding might not be: e.g. forall a. Num a => (# a, a #)
+ -- If has_sig is False, just recurse
- | otherwise
- = any is_unlifted_id (collectHsBindBinders CollNoDictBinders bind)
- where
- is_unlifted_id id = isUnliftedType (idType id)
- -- bindings always have a fixed RuntimeRep, so it's OK
- -- to call isUnliftedType here
+isUnliftedHsBind (FunBind { fun_id = L _ fun })
+ = is_unlifted_id fun
+
+isUnliftedHsBind (VarBind { var_id = var })
+ = is_unlifted_id var
+
+isUnliftedHsBind (PatBind { pat_lhs = pat })
+ = any is_unlifted_id (collectPatBinders CollNoDictBinders pat)
+ -- If we changed our view on (W3) you could add
+ -- || isUnliftedType pat_ty
+ -- to this check
+
+isUnliftedHsBind (PatSynBind {}) = panic "isUnliftedBind: PatSynBind"
+
+isUnliftedHsBinds :: LHsBinds GhcTc -> Bool
+isUnliftedHsBinds = anyBag (isUnliftedHsBind . unLoc)
+
+is_unlifted_id :: Id -> Bool
+is_unlifted_id id = isUnliftedType (idType id)
+ -- Bindings always have a fixed RuntimeRep, so it's OK
+ -- to call isUnliftedType here
-- | Is a binding a strict variable or pattern bind (e.g. @!x = ...@)?
isBangedHsBind :: HsBind GhcTc -> Bool
=====================================
compiler/GHC/HsToCore/Expr.hs
=====================================
@@ -197,7 +197,7 @@ dsUnliftedBind (FunBind { fun_id = L l fun
; let rhs' = core_wrap (mkOptTickBox tick rhs)
; return (bindNonRec fun rhs' body) }
-dsUnliftedBind (PatBind {pat_lhs = pat, pat_rhs = grhss
+dsUnliftedBind (PatBind { pat_lhs = pat, pat_rhs = grhss
, pat_ext = (ty, _) }) body
= -- let C x# y# = rhs in body
-- ==> case rhs of C x# y# -> body
=====================================
docs/users_guide/exts/primitives.rst
=====================================
@@ -136,7 +136,7 @@ There are some restrictions on the use of primitive types:
f x = let !(Foo a b, w) = ..rhs.. in ..body..
- since ``b`` has type ``Int#``.
+ since ``b`` has type ``Int#``. See :ref:`recursive-and-polymorphic-let-bindings`.
.. _unboxed-tuples:
@@ -198,7 +198,8 @@ example desugars like this:
q = snd t
in ..body..
-Indeed, the bindings can even be recursive.
+Indeed, the bindings can even be recursive. See :ref:`recursive-and-polymorphic-let-bindings`
+for a more precise account.
To refer to the unboxed tuple type constructors themselves, e.g. if you
want to attach instances to them, use ``(# #)``, ``(#,#)``, ``(#,,#)``, etc.
@@ -436,9 +437,13 @@ argument either way), GHC currently disallows the more general type
``PEither @l Int Bool -> Bool``. This is a consequence of the
`representation-polymorphic binder restriction <#representation-polymorphism-restrictions>`__,
-Due to :ghc-ticket:`19487`, it's
-currently not possible to declare levity-polymorphic data types with nullary
-data constructors. There's a workaround, though: ::
+Pattern matching against an unlifted data type work just like that for lifted
+types; but see :ref:`recursive-and-polymorphic-let-bindings` for the semantics of
+pattern bindings involving unlifted data types.
+
+Due to :ghc-ticket:`19487`, it is
+not currently possible to declare levity-polymorphic data types with nullary
+data constructors. There is a workaround, though: ::
type T :: TYPE (BoxedRep l)
data T where
=====================================
docs/users_guide/exts/strict.rst
=====================================
@@ -116,12 +116,10 @@ In both cases ``e`` is evaluated before starting to evaluate ``body``.
Note the following points:
-- This form is not the same as a bang pattern:
- The declarations ``f3 (x,y) = ...`` and ``f4 !(x,y) = ....``
- are equivalent (because the constructor pattern ``(x,y)`` forces the argument),
- but the expressions ``let (p,q) = e in body`` and ``let !(p,q) = e in body``
- are different. The former will not evaluate ``e`` unless
- ``p`` or ``q`` is forced in ``body``.
+- A strict binding (with a top level ``!``) should not be thought of as a regular
+ pattern binding that happens to have a bang pattern (:ref:`bang-patterns-informal`) on the LHS.
+ Rather, the top level ``!`` should be considered part of the let-binding, rather than
+ part of the pattern. This makes a difference when we come to the rules in :ref:`bang-patterns-sem`.
- Only a top-level bang (perhaps under parentheses) makes the binding strict; otherwise,
it is considered a normal bang pattern. For example, ::
@@ -354,8 +352,8 @@ Dynamic semantics of bang patterns
----------------------------------
The semantics of Haskell pattern matching is described in `Section
-3.17.2 <https://www.haskell.org/onlinereport/exps.html#sect3.17.2>`__ of
-the Haskell Report. To this description add one extra item 10, saying:
+3.17.2 <https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-610003.17.2>`__ of
+the Haskell Report. To this description add one extra item 9, saying:
- Matching the pattern ``!pat`` against a value ``v`` behaves as
follows:
@@ -365,8 +363,8 @@ the Haskell Report. To this description add one extra item 10, saying:
- otherwise, ``pat`` is matched against ``v``
Similarly, in Figure 4 of `Section
-3.17.3 <https://www.haskell.org/onlinereport/exps.html#sect3.17.3>`__,
-add a new case (t): ::
+3.17.3 <https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-440003.12>`__,
+add a new case (w): ::
case v of { !pat -> e; _ -> e' }
= v `seq` case v of { pat -> e; _ -> e' }
@@ -377,20 +375,62 @@ Haskell Report.
Replace the "Translation" there with the following one. Given
``let { bind1 ... bindn } in body``:
-.. admonition:: FORCE
+.. admonition:: SPLIT-LAZY
- Replace any binding ``!p = e`` with ``v = case e of p -> (x1, ..., xn); (x1, ..., xn) = v`` and replace
- ``body`` with ``v seq body``, where ``v`` is fresh. This translation works fine if
- ``p`` is already a variable ``x``, but can obviously be optimised by not
- introducing a fresh variable ``v``.
+ Given a lazy pattern binding ``p = e``, where ``p`` is not a variable,
+ and ``x1...xn`` are the variables bound by ``p``,
+ and all these binders have lifted type,
+ replace the binding with this (where ``v`` is fresh)::
-.. admonition:: SPLIT
+ v = case e of { p -> (x1, ..., xn) }
+ x1 = case v of { (x1, ..., xn) -> x1 }
+ ...
+ xn = case v of { (x1, ..., xn) -> xn }``
- Replace any binding ``p = e``, where ``p`` is not a variable, with
- ``v = e; x1 = case v of p -> x1; ...; xn = case v of p -> xn``, where
- ``v`` is fresh and ``x1``.. ``xn`` are the bound variables of ``p``.
- Again if ``e`` is a variable, this can be optimised by not introducing a
- fresh variable.
+ If n=1 (i.e. exactly one variable is bound),
+ the desugaring uses the ``Solo`` type to make a 1-tuple.
+
+.. admonition:: SPLIT-STRICT
+
+ Given a strict pattern binding ``!p = e``, where
+ ``x1...xn`` are the variables bound by ``p``,
+ and all these binders have lifted type:
+
+ 1. Replace the binding with this (where ``v`` is fresh)::
+
+ v = case e of { !p -> (x1, ..., xn) }
+ (x1, ..., xn) = v
+
+ 2. Replace ``body`` with ``v `seq` body``.
+
+ As in SPLIT-LAZY, if n=1 the desugaring uses the ``Solo`` type to make a 1-tuple.
+
+ This transformation is illegal at the top
+ level of a module (since there is no ``body``), so strict bindings are illegal at top level.
+
+ The transformation is correct when ``p`` is a variable ``x``, but can be optimised to::
+
+ let !x = e in body ==> let x = e in x `seq` body
+
+.. admonition:: CASE
+
+ Given a non-recursive strict pattern binding ``!p = e``,
+ where ``x1...xn`` are the variables bound by ``p``,
+ and any of the binders has unlifted type:
+ replace the binding with nothing at all, and replace
+ ``body`` with ``case e of p -> body``.
+
+ This transformation is illegal at the top
+ level of a module, so such bindings are rejected.
+
+ The result of this transformation is ill-scoped if any of the binders
+ ``x1...xn`` appears in ``e``; hence the restriction to non-recursive pattern bindings.
+
+ Exactly the same transformation applies to a non-recursive lazy pattern binding
+ (i.e. one lacking a top-level ``!``) that binds any unlifted variables; but
+ such a binding emits a warning :ghc-flag:`-Wunbanged-strict-patterns`. The
+ warning encourages the programmer to make visible the fact that this binding
+ is necessarily strict.
The result will be a (possibly) recursive set of bindings, binding
only simple variables on the left hand side. (One could go one step
@@ -412,56 +452,106 @@ Here is a simple non-recursive case: ::
!x = factorial y
in body
- ===> (FORCE)
- let x = factorial y in x `seq` body
+ ===> (SPLIT-STRICT)
+ let x = factorial y in x `seq` body
===> (inline seq)
- let x = factorial y in case x of x -> body
+ let x = factorial y in case x of !x -> body
===> (inline x)
- case factorial y of x -> body
+ case factorial y of !x -> body
Same again, only with a pattern binding: ::
- let !(Just x, Left y) = e in body
+ let !(Just x) = e in body
- ===> (FORCE)
- let v = case e of (Just x, Left y) -> (x,y)
- (x,y) = v
- in v `seq` body
+ ===> (SPLIT-STRICT)
+ let v = case e of !(Just x) -> Solo x
+ Solo x = v
+ in v `seq` body
- ===> (SPLIT)
- let v = case e of (Just x, Left y) -> (x,y)
- x = case v of (x,y) -> x
- y = case v of (x,y) -> y
- in v `seq` body
+ ===> (SPLIT-LAZY, drop redundant bang)
+ let v = case e of Just x -> Solo x
+ x = case v of Solo x -> x
+ in v `seq` body
===> (inline seq, float x,y bindings inwards)
- let v = case e of (Just x, Left y) -> (x,y)
- in case v of v -> let x = case v of (x,y) -> x
- y = case v of (x,y) -> y
- in body
+ let v = case e of Just x -> Solo x
+ in case v of !v -> let x = case v of Solo x -> x
+ in body
===> (fluff up v's pattern; this is a standard Core optimisation)
- let v = case e of (Just x, Left y) -> (x,y)
- in case v of v@(p,q) -> let x = case v of (x,y) -> x
- y = case v of (x,y) -> y
- in body
+ let v = case e of Just x -> Solo x
+ in case v of v@(Solo p) -> let x = case v of Solo x -> x
+ in body
===> (case of known constructor)
- let v = case e of (Just x, Left y) -> (x,y)
- in case v of v@(p,q) -> let x = p
- y = q
- in body
+ let v = case e of Just x -> Solo x
+ in case v of v@(Solo p) -> let x = p
+ in body
- ===> (inline x,y, v)
- case (case e of (Just x, Left y) -> (x,y) of
- (p,q) -> body[p/x, q/y]
+ ===> (inline x, v)
+ case (case e of Just x -> Solo x) of
+ Solo p -> body[p/x]
===> (case of case)
- case e of (Just x, Left y) -> body[p/x, q/y]
+ case e of Just x -> body[p/x]
+
+The final form is just what we want: a simple case expression. Notice, crucially,
+that that *pattern* ``Just x`` is forced eagerly, but ``x`` itself is not evaluated
+unless and until ``body`` does so. Note also that this example uses a pattern
+that binds exactly one variable, and illustrates the use of the ``Solo`` 1-tuple.
-The final form is just what we want: a simple case expression.
+Rule (SPLIT-STRICT) applies even if the pattern binds no variables::
+
+ let !(True,False) = e in body
+
+ ===> (SPLIT-STRICT)
+ let v = case e of !(True,False) -> (); () = v in v `seq` body
+
+ ===> (inline, simplify, drop redundant bang)
+ case e of (True,False) -> body
+
+That is, we force ``e`` and check that it has the right form before proceeding with ``body``.
+This happens even if the pattern is itself vacuous::
+
+ let !_ = e in body
+
+ ===> (SPLIT-STRICT)
+ let v = case e of !_ -> (); () = v in v `seq` body
+
+ ===> (inline, simplify)
+ case e of !_ -> body
+
+Again, ``e`` is forced before evaluating ``body``. This (along with ``!x = e``) is the reason
+that (SPLIT-STRICT) uses a bang-pattern in the ``case`` in the desugared right-hand side.
+
+Note that rule (CASE) applies only when any of the *binders* is unlifted;
+it is irrelevant whether the binding *itself* is unlifted (see
+`GHC proposal #35 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0035-unbanged-strict-patterns.rst>`__).
+For example (see :ref:`primitives`)::
+
+ let (# a::Int, b::Bool #) = e in body
+ ===> (SPLIT-LAZY)
+ let v = case e of (# a,b #) -> (a,b)
+ a = case v of (a,b) -> a
+ b = case v of (a,b) -> b
+ in body
+
+Even though the tuple pattern is unboxed, it is matched only when ``a`` or ``b`` are evaluated in ``body``.
+
+Here is an example with an unlifted data type::
+
+ type T :: UnliftedType
+ data T = MkT Int
+ f1 x = let MkT y = blah in body1
+ f2 x = let z :: T = blah in body2
+ f3 x = let _ :: T = blah in body3
+
+In ``f1``, even though ``T`` is an unlifted type, the pattern ``MkT y`` binds a lifted
+variable ``y``, so (SPLIT-LAZY) applies, and ``blah`` is not evaluated until ``body1`` evaluates ``y``.
+In contrast, in ``f2`` the pattern ``z :: T`` binds a variable ``z`` of unlifted type, so (CASE) applies
+and the let-binding is strict. In ``f3`` the pattern binds no variables, so again it is lazy like ``f1``.
Here is a recursive case ::
@@ -469,14 +559,14 @@ Here is a recursive case ::
!xs = factorial y : xs
in body
- ===> (FORCE)
- letrec xs = factorial y : xs in xs `seq` body
+ ===> (SPLIT-STRICT)
+ letrec xs = factorial y : xs in xs `seq` body
===> (inline seq)
- letrec xs = factorial y : xs in case xs of xs -> body
+ letrec xs = factorial y : xs in case xs of xs -> body
===> (eliminate case of value)
- letrec xs = factorial y : xs in body
+ letrec xs = factorial y : xs in body
and a polymorphic one: ::
@@ -484,10 +574,11 @@ and a polymorphic one: ::
!f = fst (reverse, True)
in body
- ===> (FORCE)
- let f = /\a. fst (reverse a, True) in f `seq` body
+ ===> (SPLIT-STRICT)
+ let f = /\a. fst (reverse a, True) in f `seq` body
+
===> (inline seq, inline f)
- case (/\a. fst (reverse a, True)) of f -> body
+ case (/\a. fst (reverse a, True)) of !f -> body
Notice that the ``seq`` is added only in the translation to Core
If we did it in Haskell source, thus ::
@@ -507,10 +598,10 @@ intuitive: ::
!f = fst (member, True)
in body
- ===> (FORCE)
- let f = /\a \(d::Eq a). fst (member, True) in f `seq` body
+ ===> (SPLIT-STRICT)
+ let f = /\a \(d::Eq a). fst (member, True) in f `seq` body
===> (inline seq, case of value)
- let f = /\a \(d::Eq a). fst (member, True) in body
+ let f = /\a \(d::Eq a). fst (member, True) in body
Note that the bang has no effect at all in this case
=====================================
testsuite/tests/deSugar/should_compile/T22719.hs
=====================================
@@ -0,0 +1,21 @@
+{-# LANGUAGE UnliftedDatatypes #-}
+{-# OPTIONS_GHC -Wall #-}
+
+module T22719 where
+
+import GHC.Exts
+
+type T :: UnliftedType
+data T = T
+
+f :: Int -> T
+f 0 = T
+f n = f (n-1)
+
+-- ex1 is lazy in (f 7)
+ex1 :: ()
+ex1 = let _ = f 7 in ()
+
+-- ex2 is strict in (f 10)
+ex2 :: ()
+ex2 = let _a = f 10 in ()
=====================================
testsuite/tests/deSugar/should_compile/T22719.stderr
=====================================
@@ -0,0 +1,30 @@
+
+==================== Tidy Core ====================
+Result size of Tidy Core
+ = {terms: 25, types: 10, coercions: 0, joins: 0/0}
+
+-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
+ex1 :: ()
+[GblId, Unf=OtherCon []]
+ex1 = GHC.Tuple.Prim.()
+
+Rec {
+-- RHS size: {terms: 15, types: 5, coercions: 0, joins: 0/0}
+f [Occ=LoopBreaker] :: Int -> T
+[GblId, Arity=1, Unf=OtherCon []]
+f = \ (ds :: Int) ->
+ case ds of wild { I# ds1 ->
+ case ds1 of {
+ __DEFAULT -> f (- @Int GHC.Num.$fNumInt wild (GHC.Types.I# 1#));
+ 0# -> T22719.T
+ }
+ }
+end Rec }
+
+-- RHS size: {terms: 6, types: 1, coercions: 0, joins: 0/0}
+ex2 :: ()
+[GblId]
+ex2 = case f (GHC.Types.I# 10#) of { T -> GHC.Tuple.Prim.() }
+
+
+
=====================================
testsuite/tests/deSugar/should_compile/all.T
=====================================
@@ -112,3 +112,4 @@ test('T16615', normal, compile, ['-ddump-ds -dsuppress-uniques'])
test('T18112', [grep_errmsg('cast')], compile, ['-ddump-ds'])
test('T19969', normal, compile, ['-ddump-simpl -dsuppress-uniques'])
test('T19883', normal, compile, [''])
+test('T22719', normal, compile, ['-ddump-simpl -dsuppress-uniques -dno-typeable-binds'])
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/6752c1bf4fb3cafc5ff002158d47b1796cfebedf
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/6752c1bf4fb3cafc5ff002158d47b1796cfebedf
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