[Git][ghc/ghc][wip/T25493] Tidy up the handling of `assert`
Simon Peyton Jones (@simonpj)
gitlab at gitlab.haskell.org
Fri Dec 13 19:28:50 UTC 2024
Simon Peyton Jones pushed to branch wip/T25493 at Glasgow Haskell Compiler / GHC
Commits:
d9e1e62d by Simon Peyton Jones at 2024-12-13T19:24:06+00:00
Tidy up the handling of `assert`
Fixes #25493
- - - - -
3 changed files:
- compiler/GHC/Tc/Gen/Head.hs
- libraries/ghc-internal/src/GHC/Internal/Base.hs
- libraries/ghc-internal/src/GHC/Internal/IO/Exception.hs
Changes:
=====================================
compiler/GHC/Tc/Gen/Head.hs
=====================================
@@ -805,32 +805,22 @@ tcCheckId name res_ty
tcInferId :: LocatedN Name -> TcM (HsExpr GhcTc, TcSigmaType)
-- Look up an occurrence of an Id
-- Do not instantiate its type
-tcInferId lname@(L _ id_name)
+tcInferId lname@(L loc id_name)
+
| id_name `hasKey` assertIdKey
- = do { dflags <- getDynFlags
+ = -- See Note [Overview of assertions]
+ do { dflags <- getDynFlags
; if gopt Opt_IgnoreAsserts dflags
then tc_infer_id lname
- else tc_infer_assert lname }
+ else tc_infer_id (L loc assertErrorName) }
| otherwise
- = do { (expr, ty) <- tc_infer_id lname
- ; traceTc "tcInferId" (ppr id_name <+> dcolon <+> ppr ty)
- ; return (expr, ty) }
-
-tc_infer_assert :: LocatedN Name -> TcM (HsExpr GhcTc, TcSigmaType)
--- Deal with an occurrence of 'assert'
--- See Note [Adding the implicit parameter to 'assert']
-tc_infer_assert (L loc assert_name)
- = do { assert_error_id <- tcLookupId assertErrorName
- ; (wrap, id_rho) <- topInstantiate (OccurrenceOf assert_name)
- (idType assert_error_id)
- ; return (mkHsWrap wrap (HsVar noExtField (L loc assert_error_id)), id_rho)
- }
+ = tc_infer_id lname
tc_infer_id :: LocatedN Name -> TcM (HsExpr GhcTc, TcSigmaType)
tc_infer_id (L loc id_name)
= do { thing <- tcLookup id_name
- ; case thing of
+ ; (expr,ty) <- case thing of
ATcId { tct_id = id }
-> do { check_local_id id
; return_id id }
@@ -845,12 +835,45 @@ tc_infer_id (L loc id_name)
(tcTyThingTyCon_maybe -> Just tc) -> failIllegalTyCon WL_Anything (tyConName tc)
ATyVar name _ -> failIllegalTyVal name
- _ -> failWithTc $ TcRnExpectedValueId thing }
+ _ -> failWithTc $ TcRnExpectedValueId thing
+
+ ; traceTc "tcInferId" (ppr id_name <+> dcolon <+> ppr ty)
+ ; return (expr, ty) }
where
return_id id = return (HsVar noExtField (L loc id), idType id)
-{- Note [Suppress hints with RequiredTypeArguments]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+{- Note [Overview of assertions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If you write (assert pred x) then
+
+ * If `-fignore-asserts` (which sets Opt_IgnoreAsserts) is on, the code is
+ typechecked as written, but `assert`, defined in GHC.Internal.Base
+ assert _pred r = r
+ simply ignores `pred`
+
+ * But without `-fignore-asserts`, GHC rewrites it to (assertError pred e)
+ and that is defined in GHC.Internal.IO.Exception as
+ assertError :: (?callStack :: CallStack) => Bool -> a -> a
+ which does test the predicate and, if it is not True, throws an exception,
+ capturing the CallStack.
+
+ This rewrite is done in `tcInferId`.
+
+So `-fignore-asserts` makes the assertion go away altogether, which may be good for
+production code.
+
+The reason that `assert` and `assertError` are defined in very different modules
+is a historical accident.
+
+Note: the Haddock for `assert` is on `GHC.Internal.Base.assert`, since that is
+what appears in the user's source proram.
+
+It's not entirely kosher to rewrite `assert` to `assertError`, because there's no
+way to "undo" if you want to see the original source code in the typechecker
+output. We can fix this if it becomes a problem.
+
+Note [Suppress hints with RequiredTypeArguments]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When a type variable is used at the term level, GHC assumes the user might
have made a typo and suggests a term variable with a similar name.
@@ -948,16 +971,8 @@ tcInferPatSyn ps
nonBidirectionalErr :: Name -> TcRnMessage
nonBidirectionalErr = TcRnPatSynNotBidirectional
-{- Note [Adding the implicit parameter to 'assert']
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The typechecker transforms (assert e1 e2) to (assertError e1 e2).
-This isn't really the Right Thing because there's no way to "undo"
-if you want to see the original source code in the typechecker
-output. We'll have fix this in due course, when we care more about
-being able to reconstruct the exact original program.
-
-Note [Typechecking data constructors]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+{- Note [Typechecking data constructors]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
As per Note [Polymorphisation of linear fields] in
GHC.Core.Multiplicity, linear fields of data constructors get a
polymorphic multiplicity when the data constructor is used as a term:
=====================================
libraries/ghc-internal/src/GHC/Internal/Base.hs
=====================================
@@ -2107,6 +2107,9 @@ id x = x
-- Assertion function. This simply ignores its boolean argument.
-- The compiler may rewrite it to @('assertError' line)@.
+-- The Haddock below is attached to `assert`, since that is
+-- what occurs in source programs.
+-- See Note [Overview of assertions] in GHC.Tc.Gen.Head
-- | If the first argument evaluates to 'True', then the result is the
-- second argument. Otherwise an 'Control.Exception.AssertionFailed' exception
@@ -2115,14 +2118,9 @@ id x = x
--
-- Assertions can normally be turned on or off with a compiler flag
-- (for GHC, assertions are normally on unless optimisation is turned on
--- with @-O@ or the @-fignore-asserts@
--- option is given). When assertions are turned off, the first
--- argument to 'assert' is ignored, and the second argument is
--- returned as the result.
-
--- SLPJ: in 5.04 etc 'assert' is in GHC.Prim,
--- but from Template Haskell onwards it's simply
--- defined here in Base.hs
+-- with @-O@ or the @-fignore-asserts@ option is given). When assertions
+-- are turned off, the first argument to 'assert' is ignored, and the second
+-- argument is returned as the result.
assert :: Bool -> a -> a
assert _pred r = r
=====================================
libraries/ghc-internal/src/GHC/Internal/IO/Exception.hs
=====================================
@@ -438,6 +438,7 @@ instance Show IOException where
_ -> showString " (" . showString s . showString ")")
assertError :: (?callStack :: CallStack) => Bool -> a -> a
+-- See Note [Overview of assertions] in GHC.Tc.Gen.Head
assertError predicate v
| predicate = v
| otherwise = lazy $ unsafeDupablePerformIO $ do -- lazy: See Note [Strictness of assertError]
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d9e1e62d29067b2243b60c4a17807c1b9bd694a4
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/d9e1e62d29067b2243b60c4a17807c1b9bd694a4
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