[Git][ghc/ghc][wip/T22719] Document the semantics of pattern bindings a bit better

Simon Peyton Jones (@simonpj) gitlab at gitlab.haskell.org
Tue Jan 10 22:03:28 UTC 2023



Simon Peyton Jones pushed to branch wip/T22719 at Glasgow Haskell Compiler / GHC


Commits:
dae63714 by Simon Peyton Jones at 2023-01-10T22:03:56+00:00
Document the semantics of pattern bindings a bit better

This MR is in response to the discussion on #22719

- - - - -


6 changed files:

- compiler/GHC/Hs/Utils.hs
- compiler/GHC/HsToCore/Expr.hs
- 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/strict.rst
=====================================
@@ -377,21 +377,36 @@ Haskell Report.
 Replace the "Translation" there with the following one.  Given
 ``let { bind1 ... bindn } in body``:
 
-.. admonition:: FORCE
-
-    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``.
-
 .. admonition:: SPLIT
 
-    Replace any binding ``p = e``, where ``p`` is not a variable, with
+    Given a binding ``p = e``, where ``p`` is not a variable or a bang pattern,
+    where ``x1...xn`` are the variables bound by ``p``,
+    and all these binders have lifted type:
+    replace the binding 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.
 
+.. admonition:: FORCE
+
+    Given a binding binding ``!p = e``, where ``x1...xn`` are the variables bound by ``p``,
+    and all these binders have lifted type:
+    replace the binding 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``.  This transformation is illegal at the top
+    level of a module (since there is no ``body``), so such bindings are rejected.
+
+.. admonition:: CASE
+
+    Given a binding 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 applies even if ``p`` binds no variables at all.
+    This transformation is illegal at the top
+    level of a module, so such bindings are rejected.
+
 The result will be a (possibly) recursive set of bindings, binding
 only simple variables on the left hand side. (One could go one step
 further, as in the Haskell Report and make the recursive bindings


=====================================
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/dae6371493faef399011758753f13fd53e365bac

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/dae6371493faef399011758753f13fd53e365bac
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