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diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst new file mode 100644 index 0000000000..0b3faa4105 --- /dev/null +++ b/docs/users_guide/using-warnings.rst @@ -0,0 +1,861 @@ +.. _options-sanity: + +Warnings and sanity-checking +---------------------------- + +.. index:: + single: sanity-checking options + single: warnings + +GHC has a number of options that select which types of non-fatal error +messages, otherwise known as warnings, can be generated during +compilation. By default, you get a standard set of warnings which are +generally likely to indicate bugs in your program. These are: +``-fwarn-overlapping-patterns``, ``-fwarn-warnings-deprecations``, +``-fwarn-deprecated-flags``, ``-fwarn-unrecognised-pragmas``, +``-fwarn-missed-specialisations``, ``-fwarn-duplicate-constraints``, +``-fwarn-duplicate-exports``, ``-fwarn-overflowed-literals``, +``-fwarn-empty-enumerations``, ``-fwarn-missing-fields``, +``-fwarn-missing-methods``, ``-fwarn-wrong-do-bind``, +``-fwarn-unsupported-calling-conventions``, +``-fwarn-dodgy-foreign-imports``, ``-fwarn-inline-rule-shadowing``, +``-fwarn-unsupported-llvm-version``, ``-fwarn-context-quantification``, +and ``-fwarn-tabs``. The following flags are simple ways to select +standard “packages” of warnings: + +``-W`` + .. index:: + single: -W option + + Provides the standard warnings plus ``-fwarn-unused-binds``, + ``-fwarn-unused-matches``, ``-fwarn-unused-imports``, + ``-fwarn-incomplete-patterns``, ``-fwarn-dodgy-exports``, and + ``-fwarn-dodgy-imports``. + +``-Wall`` + .. index:: + single: -Wall + + Turns on all warning options that indicate potentially suspicious + code. The warnings that are *not* enabled by ``-Wall`` are + ``-fwarn-incomplete-uni-patterns``, + ``-fwarn-incomplete-record-updates``, + ``-fwarn-monomorphism-restriction``, ``-fwarn-auto-orphans``, + ``-fwarn-implicit-prelude``, ``-fwarn-missing-local-sigs``, + ``-fwarn-missing-exported-sigs``, ``-fwarn-missing-import-lists`` + and ``-fwarn-identities``. + +``-w`` + .. index:: + single: -w + + Turns off all warnings, including the standard ones and those that + ``-Wall`` doesn't enable. + +``-Werror`` + .. index:: + single: -Werror + + Makes any warning into a fatal error. Useful so that you don't miss + warnings when doing batch compilation. + +``-Wwarn`` + .. index:: + single: -Wwarn + + Warnings are treated only as warnings, not as errors. This is the + default, but can be useful to negate a ``-Werror`` flag. + +The full set of warning options is described below. To turn off any +warning, simply give the corresponding ``-fno-warn-...`` option on the +command line. + +``-fwarn-typed-holes`` + .. index:: + single: -fwarn-typed-holes + single: warnings + + Determines whether the compiler reports typed holes warnings. Has no + effect unless typed holes errors are deferred until runtime. See + :ref:`typed-holes` and :ref:`defer-type-errors` + + This warning is on by default. + +``-fwarn-type-errors`` + .. index:: + single: -fwarn-type-errors + single: warnings + + Causes a warning to be reported when a type error is deferred until + runtime. See :ref:`defer-type-errors` + + This warning is on by default. + +``-fdefer-type-errors`` + .. index:: + single: -fdefer-type-errors + single: warnings + + Defer as many type errors as possible until runtime. At compile time + you get a warning (instead of an error). At runtime, if you use a + value that depends on a type error, you get a runtime error; but you + can run any type-correct parts of your code just fine. See + :ref:`defer-type-errors` + +``-fdefer-typed-holes`` + .. index:: + single: -fdefer-typed-holes + single: warnings + + Defer typed holes errors until runtime. This will turn the errors + produced by :ref:`typed holes <typed-holes>` into warnings. Using a value + that depends on a typed hole produces a runtime error, the same as + ``-fdefer-type-errors`` (which implies this option). See :ref:`typed-holes` + and :ref:`defer-type-errors`. + + Implied by ``-fdefer-type-errors``. See also ``-fwarn-typed-holes``. + +``-fwarn-partial-type-signatures`` + .. index:: + single: -fwarn-partial-type-signatures + single: warnings + + Determines whether the compiler reports holes in partial type + signatures as warnings. Has no effect unless + ``-XPartialTypeSignatures`` is enabled, which controls whether + errors should be generated for holes in types or not. See + :ref:`partial-type-signatures`. + + This warning is on by default. + +``-fhelpful-errors`` + .. index:: + single: -fhelpful-errors + single: warnings + + When a name or package is not found in scope, make suggestions for + the name or package you might have meant instead. + + This option is on by default. + +``-fwarn-unrecognised-pragmas`` + .. index:: + single: -fwarn-unrecognised-pragmas + single: warnings + single: pragmas + + Causes a warning to be emitted when a pragma that GHC doesn't + recognise is used. As well as pragmas that GHC itself uses, GHC also + recognises pragmas known to be used by other tools, e.g. + ``OPTIONS_HUGS`` and ``DERIVE``. + + This option is on by default. + +``-fwarn-missed-specialisations``, ``-fwarn-all-missed-specialisations`` + .. index:: + single: -fwarn-missed-specialisations + single: -fwarn-all-missed-specialisations + single: warnings + single: pragmas + + Emits a warning if GHC cannot specialise a function that is imported + and overloaded, usually because the function needs an ``INLINEABLE`` + pragma.. The "all" form reports all such situations. The "non-all" + form only reports when the situation arises during specialisation of + an imported function; presumably teh latter was marked + ``INLINEABLE`` so that it would specialise but if it, in turn, calls + other functions that are not specialised you won't get the + performance boost you expect. + + ``-fwarn-missed-specialisations`` is on by default; + ``-fwarn-all-missed-specialisations`` is implied by ``-Wall``. + +``-fwarn-warnings-deprecations`` + .. index:: + single: -fwarn-warnings-deprecations + single: warnings + single: deprecations + + Causes a warning to be emitted when a module, function or type with + a ``WARNING`` or ``DEPRECATED pragma`` is used. See + :ref:`warning-deprecated-pragma` for more details on the pragmas. + + This option is on by default. + +``-fwarn-amp`` + .. index:: + single: -fwarn-amp + single: AMP + single: Applicative-Monad Proposal + + Causes a warning to be emitted when a definition is in conflict with + the AMP (Applicative-Monad proosal), namely: 1. Instance of Monad + without Applicative; 2. Instance of MonadPlus without Alternative; + 3. Custom definitions of join/pure/<\*> + + This option is on by default. + +``-fwarn-deprecated-flags`` + .. index:: + single: -fwarn-deprecated-flags + single: deprecated-flags + + Causes a warning to be emitted when a deprecated command-line flag + is used. + + This option is on by default. + +``-fwarn-unsupported-calling-conventions`` + .. index:: + single: -fwarn-unsupported-calling-conventions + + Causes a warning to be emitted for foreign declarations that use + unsupported calling conventions. In particular, if the ``stdcall`` + calling convention is used on an architecture other than i386 then + it will be treated as ``ccall``. + +``-fwarn-dodgy-foreign-imports`` + .. index:: + single: -fwarn-dodgy-foreign-imports + + Causes a warning to be emitted for foreign imports of the following + form: + + :: + + foreign import "f" f :: FunPtr t + + on the grounds that it probably should be + + :: + + foreign import "&f" f :: FunPtr t + + The first form declares that \`f\` is a (pure) C function that takes + no arguments and returns a pointer to a C function with type \`t\`, + whereas the second form declares that \`f\` itself is a C function + with type \`t\`. The first declaration is usually a mistake, and one + that is hard to debug because it results in a crash, hence this + warning. + +``-fwarn-dodgy-exports`` + .. index:: + single: -fwarn-dodgy-exports + + Causes a warning to be emitted when a datatype ``T`` is exported + with all constructors, i.e. ``T(..)``, but is it just a type + synonym. + + Also causes a warning to be emitted when a module is re-exported, + but that module exports nothing. + +``-fwarn-dodgy-imports`` + .. index:: + single: -fwarn-dodgy-imports + + Causes a warning to be emitted in the following cases: + + - When a datatype ``T`` is imported with all constructors, i.e. + ``T(..)``, but has been exported abstractly, i.e. ``T``. + + - When an ``import`` statement hides an entity that is not + exported. + +``-fwarn-overflowed-literals`` + .. index:: + single: -fwarn-overflowed-literals + + Causes a warning to be emitted if a literal will overflow, e.g. + ``300 :: Word8``. + +``-fwarn-empty-enumerations`` + .. index:: + single: -fwarn-empty-enumerations + + Causes a warning to be emitted if an enumeration is empty, e.g. + ``[5 .. 3]``. + +``-fwarn-lazy-unlifted-bindings`` + .. index:: + single: -fwarn-lazy-unlifted-bindings + + This flag is a no-op, and will be removed in GHC 7.10. + +``-fwarn-duplicate-constraints`` + .. index:: + single: -fwarn-duplicate-constraints + single: duplicate constraints, warning + + Have the compiler warn about duplicate constraints in a type + signature. For example + + :: + + f :: (Eq a, Show a, Eq a) => a -> a + + The warning will indicate the duplicated ``Eq a`` constraint. + + This option is now deprecated in favour of + ``-fwarn-redundant-constraints``. + +``-fwarn-redundant-constraints`` + .. index:: + single: -fwarn-redundant-constraints + single: redundant constraints, warning + + Have the compiler warn about redundant constraints in a type + signature. In particular: + + - A redundant constraint within the type signature itself: + + :: + + f :: (Eq a, Ord a) => a -> a + + The warning will indicate the redundant ``Eq a`` constraint: it + is subsumed by the ``Ord a`` constraint. + + - A constraint in the type signature is not used in the code it + covers: + + :: + + f :: Eq a => a -> a -> Bool + f x y = True + + The warning will indicate the redundant ``Eq a`` constraint: : it + is not used by the definition of ``f``.) + + Similar warnings are given for a redundant constraint in an instance + declaration. + + This option is on by default. As usual you can suppress it on a + per-module basis with ``-fno-warn-redundant-constraints``. + Occasionally you may specifically want a function to have a more + constrained signature than necessary, perhaps to leave yourself + wiggle-room for changing the implementation without changing the + API. In that case, you can suppress the warning on a per-function + basis, using a call in a dead binding. For example: + + :: + + f :: Eq a => a -> a -> Bool + f x y = True + where + _ = x == x -- Suppress the redundant-constraint warning for (Eq a) + + Here the call to ``(==)`` makes GHC think that the ``(Eq a)`` + constraint is needed, so no warning is issued. + +``-fwarn-duplicate-exports`` + .. index:: + single: -fwarn-duplicate-exports + single: duplicate exports, warning + single: export lists, duplicates + + Have the compiler warn about duplicate entries in export lists. This + is useful information if you maintain large export lists, and want + to avoid the continued export of a definition after you've deleted + (one) mention of it in the export list. + + This option is on by default. + +``-fwarn-hi-shadowing`` + .. index:: + single: -fwarn-hi-shadowing + single: shadowing; interface files + + Causes the compiler to emit a warning when a module or interface + file in the current directory is shadowing one with the same module + name in a library or other directory. + +``-fwarn-identities`` + .. index:: + single: -fwarn-identities + + Causes the compiler to emit a warning when a Prelude numeric + conversion converts a type T to the same type T; such calls are + probably no-ops and can be omitted. The functions checked for are: + ``toInteger``, ``toRational``, ``fromIntegral``, and ``realToFrac``. + +``-fwarn-implicit-prelude`` + .. index:: + single: -fwarn-implicit-prelude + single: implicit prelude, warning + + Have the compiler warn if the Prelude is implicitly imported. This + happens unless either the Prelude module is explicitly imported with + an ``import ... Prelude ...`` line, or this implicit import is + disabled (either by ``-XNoImplicitPrelude`` or a + ``LANGUAGE NoImplicitPrelude`` pragma). + + Note that no warning is given for syntax that implicitly refers to + the Prelude, even if ``-XNoImplicitPrelude`` would change whether it + refers to the Prelude. For example, no warning is given when ``368`` + means ``Prelude.fromInteger (368::Prelude.Integer)`` (where + ``Prelude`` refers to the actual Prelude module, regardless of the + imports of the module being compiled). + + This warning is off by default. + +``-fwarn-incomplete-patterns``, ``-fwarn-incomplete-uni-patterns`` + .. index:: + single: -fwarn-incomplete-patterns + single: -fwarn-incomplete-uni-patterns + single: incomplete patterns, warning + single: patterns, incomplete + + The option ``-fwarn-incomplete-patterns`` warns about places where a + pattern-match might fail at runtime. The function ``g`` below will + fail when applied to non-empty lists, so the compiler will emit a + warning about this when ``-fwarn-incomplete-patterns`` is enabled. + + :: + + g [] = 2 + + This option isn't enabled by default because it can be a bit noisy, + and it doesn't always indicate a bug in the program. However, it's + generally considered good practice to cover all the cases in your + functions, and it is switched on by ``-W``. + + The flag ``-fwarn-incomplete-uni-patterns`` is similar, except that + it applies only to lambda-expressions and pattern bindings, + constructs that only allow a single pattern: + + :: + + h = \[] -> 2 + Just k = f y + +``-fwarn-incomplete-record-updates`` + .. index:: + single: -fwarn-incomplete-record-updates + single: incomplete record updates, warning + single: record updates, incomplete + + The function ``f`` below will fail when applied to ``Bar``, so the + compiler will emit a warning about this when + ``-fwarn-incomplete-record-updates`` is enabled. + + :: + + data Foo = Foo { x :: Int } + | Bar + + f :: Foo -> Foo + f foo = foo { x = 6 } + + This option isn't enabled by default because it can be very noisy, + and it often doesn't indicate a bug in the program. + +``-fwarn-missing-fields`` + .. index:: + single: -fwarn-missing-fields + single: missing fields, warning + single: fields, missing + + This option is on by default, and warns you whenever the + construction of a labelled field constructor isn't complete, missing + initialisers for one or more fields. While not an error (the missing + fields are initialised with bottoms), it is often an indication of a + programmer error. + +``-fwarn-missing-import-lists`` + .. index:: + single: -fwarn-import-lists + single: missing import lists, warning + single: import lists, missing + + This flag warns if you use an unqualified ``import`` declaration + that does not explicitly list the entities brought into scope. For + example + + :: + + module M where + import X( f ) + import Y + import qualified Z + p x = f x x + + The ``-fwarn-import-lists`` flag will warn about the import of ``Y`` + but not ``X`` If module ``Y`` is later changed to export (say) + ``f``, then the reference to ``f`` in ``M`` will become ambiguous. + No warning is produced for the import of ``Z`` because extending + ``Z``\'s exports would be unlikely to produce ambiguity in ``M``. + +``-fwarn-missing-methods`` + .. index:: + single: -fwarn-missing-methods + single: missing methods, warning + single: methods, missing + + This option is on by default, and warns you whenever an instance + declaration is missing one or more methods, and the corresponding + class declaration has no default declaration for them. + + The warning is suppressed if the method name begins with an + underscore. Here's an example where this is useful: + + :: + + class C a where + _simpleFn :: a -> String + complexFn :: a -> a -> String + complexFn x y = ... _simpleFn ... + + The idea is that: (a) users of the class will only call + ``complexFn``; never ``_simpleFn``; and (b) instance declarations + can define either ``complexFn`` or ``_simpleFn``. + + The ``MINIMAL`` pragma can be used to change which combination of + methods will be required for instances of a particular class. See + :ref:`minimal-pragma`. + +``-fwarn-missing-signatures`` + .. index:: + single: -fwarn-missing-signatures + single: type signatures, missing + + If you would like GHC to check that every top-level function/value + has a type signature, use the ``-fwarn-missing-signatures`` option. + As part of the warning GHC also reports the inferred type. The + option is off by default. + +``-fwarn-missing-exported-sigs`` + .. index:: + single: -fwarn-missing-exported-sigs + single: type signatures, missing + + If you would like GHC to check that every exported top-level + function/value has a type signature, but not check unexported + values, use the ``-fwarn-missing-exported-sigs`` option. This option + takes precedence over ``-fwarn-missing-signatures``. As part of the + warning GHC also reports the inferred type. The option is off by + default. + +``-fwarn-missing-local-sigs`` + .. index:: + single: -fwarn-missing-local-sigs + single: type signatures, missing + + If you use the ``-fwarn-missing-local-sigs`` flag GHC will warn you + about any polymorphic local bindings. As part of the warning GHC + also reports the inferred type. The option is off by default. + +``-fwarn-name-shadowing`` + .. index:: + single: -fwarn-name-shadowing + single: shadowing, warning + + This option causes a warning to be emitted whenever an inner-scope + value has the same name as an outer-scope value, i.e. the inner + value shadows the outer one. This can catch typographical errors + that turn into hard-to-find bugs, e.g., in the inadvertent capture + of what would be a recursive call in + ``f = ... let f = id in ... f ...``. + + The warning is suppressed for names beginning with an underscore. + For example + + :: + + f x = do { _ignore <- this; _ignore <- that; return (the other) } + +``-fwarn-orphans, -fwarn-auto-orphans`` + .. index:: + single: -fwarn-orphans + single: -fwarn-auto-orphans + single: orphan instances, warning + single: orphan rules, warning + + These flags cause a warning to be emitted whenever the module + contains an "orphan" instance declaration or rewrite rule. An + instance declaration is an orphan if it appears in a module in which + neither the class nor the type being instanced are declared in the + same module. A rule is an orphan if it is a rule for a function + declared in another module. A module containing any orphans is + called an orphan module. + + The trouble with orphans is that GHC must pro-actively read the + interface files for all orphan modules, just in case their instances + or rules play a role, whether or not the module's interface would + otherwise be of any use. See :ref:`orphan-modules` for details. + + The flag ``-fwarn-orphans`` warns about user-written orphan rules or + instances. The flag ``-fwarn-auto-orphans`` warns about + automatically-generated orphan rules, notably as a result of + specialising functions, for type classes (``Specialise``) or + argument values (``-fspec-constr``). + +``-fwarn-overlapping-patterns`` + .. index:: + single: -fwarn-overlapping-patterns + single: overlapping patterns, warning + single: patterns, overlapping + + By default, the compiler will warn you if a set of patterns are + overlapping, e.g., + + :: + + f :: String -> Int + f [] = 0 + f (_:xs) = 1 + f "2" = 2 + + where the last pattern match in ``f`` won't ever be reached, as the + second pattern overlaps it. More often than not, redundant patterns + is a programmer mistake/error, so this option is enabled by default. + +``-fwarn-tabs`` + .. index:: + single: -fwarn-tabs + single: tabs, warning + + Have the compiler warn if there are tabs in your source file. + +``-fwarn-type-defaults`` + .. index:: + single: -fwarn-type-defaults + single: defaulting mechanism, warning + + Have the compiler warn/inform you where in your source the Haskell + defaulting mechanism for numeric types kicks in. This is useful + information when converting code from a context that assumed one + default into one with another, e.g., the ‘default default’ for + Haskell 1.4 caused the otherwise unconstrained value ``1`` to be + given the type ``Int``, whereas Haskell 98 and later defaults it to + ``Integer``. This may lead to differences in performance and + behaviour, hence the usefulness of being non-silent about this. + + This warning is off by default. + +``-fwarn-monomorphism-restriction`` + .. index:: + single: -fwarn-monomorphism-restriction + single: monomorphism restriction, warning + + Have the compiler warn/inform you where in your source the Haskell + Monomorphism Restriction is applied. If applied silently the MR can + give rise to unexpected behaviour, so it can be helpful to have an + explicit warning that it is being applied. + + This warning is off by default. + +``-fwarn-unticked-promoted-constructors`` + .. index:: + single: -fwarn-unticked-promoted-constructors + single: promoted constructor, warning + + Warn if a promoted data constructor is used without a tick preceding + its name. + + For example: + + :: + + data Nat = Succ Nat | Zero + + data Vec n s where + Nil :: Vec Zero a + Cons :: a -> Vec n a -> Vec (Succ n) a + + Will raise two warnings because ``Zero`` and ``Succ`` are not + written as ``'Zero`` and ``'Succ``. + + This warning is is enabled by default in ``-Wall`` mode. + +``-fwarn-unused-binds`` + .. index:: + single: -fwarn-unused-binds + single: unused binds, warning + single: binds, unused + + Report any function definitions (and local bindings) which are + unused. An alias for + + - ``-fwarn-unused-top-binds`` + - ``-fwarn-unused-local-binds`` + - ``-fwarn-unused-pattern-binds`` + +``-fwarn-unused-top-binds`` + .. index:: + single: -fwarn-unused-top-binds + single: unused binds, warning + single: binds, unused + + Report any function definitions which are unused. + + More precisely, warn if a binding brings into scope a variable that + is not used, except if the variable's name starts with an + underscore. The "starts-with-underscore" condition provides a way to + selectively disable the warning. + + A variable is regarded as "used" if + + - It is exported, or + + - It appears in the right hand side of a binding that binds at + least one used variable that is used + + For example + + :: + + module A (f) where + f = let (p,q) = rhs1 in t p -- No warning: q is unused, but is locally bound + t = rhs3 -- No warning: f is used, and hence so is t + g = h x -- Warning: g unused + h = rhs2 -- Warning: h is only used in the + -- right-hand side of another unused binding + _w = True -- No warning: _w starts with an underscore + +``-fwarn-unused-local-binds`` + .. index:: + single: -fwarn-unused-local-binds + single: unused binds, warning + single: binds, unused + + Report any local definitions which are unused. For example + + :: + + module A (f) where + f = let (p,q) = rhs1 in t p -- Warning: q is unused + g = h x -- No warning: g is unused, but is a top-level binding + +``-fwarn-unused-pattern-binds`` + .. index:: + single: -fwarn-unused-pattern-binds + single: unused binds, warning + single: binds, unused + + Warn if a pattern binding binds no variables at all, unless it is a + lone, possibly-banged, wild-card pattern. For example: + + :: + + Just _ = rhs3 -- Warning: unused pattern binding + (_, _) = rhs4 -- Warning: unused pattern binding + _ = rhs3 -- No warning: lone wild-card pattern + !_ = rhs4 -- No warning: banged wild-card pattern; behaves like seq + + The motivation for allowing lone wild-card patterns is they are not + very different from ``_v = rhs3``, which elicits no warning; and + they can be useful to add a type constraint, e.g. ``_ = x::Int``. A + lone banged wild-card pattern is useful as an alternative (to + ``seq``) way to force evaluation. + +``-fwarn-unused-imports`` + .. index:: + single: -fwarn-unused-imports + single: unused imports, warning + single: imports, unused + + Report any modules that are explicitly imported but never used. + However, the form ``import M()`` is never reported as an unused + import, because it is a useful idiom for importing instance + declarations, which are anonymous in Haskell. + +``-fwarn-unused-matches`` + .. index:: + single: -fwarn-unused-matches + single: unused matches, warning + single: matches, unused + + Report all unused variables which arise from pattern matches, + including patterns consisting of a single variable. For instance + ``f x y = []`` would report ``x`` and ``y`` as unused. The warning + is suppressed if the variable name begins with an underscore, thus: + + :: + + f _x = True + +``-fwarn-unused-do-bind`` + .. index:: + single: -fwarn-unused-do-bind + single: unused do binding, warning + single: do binding, unused + + Report expressions occurring in ``do`` and ``mdo`` blocks that + appear to silently throw information away. For instance + ``do { mapM popInt xs ; return 10 }`` would report the first + statement in the ``do`` block as suspicious, as it has the type + ``StackM [Int]`` and not ``StackM ()``, but that ``[Int]`` value is + not bound to anything. The warning is suppressed by explicitly + mentioning in the source code that your program is throwing + something away: + + :: + + do { _ <- mapM popInt xs ; return 10 } + + Of course, in this particular situation you can do even better: + + :: + + do { mapM_ popInt xs ; return 10 } + +``-fwarn-context-quantification`` + .. index:: + single: -fwarn-context-quantification + single: implicit context quantification, warning + single: context, implicit quantification + + Report if a variable is quantified only due to its presence in a + context (see :ref:`universal-quantification`). For example, + + :: + + type T a = Monad m => a -> f a + + It is recommended to write this polymorphic type as + + :: + + type T a = forall m. Monad m => a -> f a + + instead. + +``-fwarn-wrong-do-bind`` + .. index:: + single: -fwarn-wrong-do-bind + single: apparently erroneous do binding, warning + single: do binding, apparently erroneous + + Report expressions occurring in ``do`` and ``mdo`` blocks that + appear to lack a binding. For instance + ``do { return (popInt 10) ; return 10 }`` would report the first + statement in the ``do`` block as suspicious, as it has the type + ``StackM (StackM Int)`` (which consists of two nested applications + of the same monad constructor), but which is not then "unpacked" by + binding the result. The warning is suppressed by explicitly + mentioning in the source code that your program is throwing + something away: + + :: + + do { _ <- return (popInt 10) ; return 10 } + + For almost all sensible programs this will indicate a bug, and you + probably intended to write: + + :: + + do { popInt 10 ; return 10 } + +``-fwarn-inline-rule-shadowing`` + .. index:: + single: -fwarn-inline-rule-shadowing + + Warn if a rewrite RULE might fail to fire because the function might + be inlined before the rule has a chance to fire. See + :ref:`rules-inline`. + +If you're feeling really paranoid, the ``-dcore-lint`` option is a good choice. +It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's +sanity, not yours.) + |