bugs.rst

.. _bugs-and-infelicities:

Known bugs and infelicities
===========================

.. _vs-Haskell-defn:

Haskell standards vs. Glasgow Haskell: language non-compliance
--------------------------------------------------------------

.. index::
single: GHC vs the Haskell standards
single: Haskell standards vs GHC

This section lists Glasgow Haskell infelicities in its implementation of
Haskell 98 and Haskell 2010. See also the “when things go wrong” section
(:ref:`wrong`) for information about crashes, space leaks, and other
undesirable phenomena.

The limitations here are listed in Haskell Report order (roughly).

.. _haskell-standards-divergence:

Divergence from Haskell 98 and Haskell 2010
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GHC aims to be able to behave (mostly) like a Haskell 98 or Haskell 2010
compiler, if you tell it to try to behave like that with the
:extension:`Haskell98` and :extension:`Haskell2010` flags. The known deviations
from the standards are described below. Unless otherwise stated, the deviation
applies in both Haskell 98 and Haskell 2010 mode.

.. _infelicities-lexical:

Lexical syntax
^^^^^^^^^^^^^^

- Certain lexical rules regarding qualified identifiers are slightly
different in GHC compared to the Haskell report. When you have
⟨module⟩\ ``.``\ ⟨reservedop⟩, such as ``M.\``, GHC will interpret it
as a single qualified operator rather than the two lexemes ``M`` and
``.\``.

- ``forall`` is always a reserved keyword at the type level, contrary
to the Haskell Report, which allows type variables to be named ``forall``.
Note that this does not imply that GHC always enables the
:extension:`ExplicitForAll` extension. Even without this extension enabled,
reserving ``forall`` as a keyword has significance. For instance, GHC will
not parse the type signature ``foo :: forall x``.

- The ``(!)`` operator, when written in prefix form (preceded by whitespace
and not followed by whitespace, as in ``f !x = ...``), is interpreted as a
bang pattern, contrary to the Haskell Report, which prescribes to treat ``!``
as an operator regardless of surrounding whitespace. Note that this does not
imply that GHC always enables :extension:`BangPatterns`. Without the
extension, GHC will issue a parse error on ``f !x``, asking to enable the
extension.

- Irrefutable patterns must be written in prefix form::

f ~a ~b = ... -- accepted by both GHC and the Haskell Report
f ~ a ~ b = ... -- accepted by the Haskell Report but not GHC

When written in non-prefix form, ``(~)`` is treated by GHC as a regular
infix operator.

See `GHC Proposal #229 `__
for the precise rules.

- Strictness annotations in data declarations must be written in prefix form::

data T = MkT !Int -- accepted by both GHC and the Haskell Report
data T = MkT ! Int -- accepted by the Haskell Report but not GHC

See `GHC Proposal #229 `__
for the precise rules.

- As-patterns must not be surrounded by whitespace on either side::

f p@(x, y, z) = ... -- accepted by both GHC and the Haskell Report

-- accepted by the Haskell Report but not GHC:
f p @ (x, y, z) = ...
f p @(x, y, z) = ...
f p@ (x, y, z) = ...

When surrounded by whitespace on both sides, ``(@)`` is treated by GHC as a
regular infix operator.

When preceded but not followed by whitespace, ``(@)`` is treated as a
visible type application.

See `GHC Proposal #229 `__
for the precise rules.

- Haskell Report allows any Unicode Decimal Number in decimal literals.
However, GHC accepts only ASCII numbers::

ascDigit → 0 | 1 | … | 9
decimal → ascDigit {ascDigit}

- GHC is more lenient in which characters are allowed in the identifiers.
Unicode Other Letters are considered to be small letters,
therefore variable identifiers can begin with them.
Digit class contains all Unicode numbers instead of just Decimal Numbers.
Modifier Letters and Non-Spacing Marks can appear in the tail
of the identifiers.::

uniSmall → any Unicode Lowercase Letter or Other Letter
uniDigit → any Unicode Decimal Number, Letter Number or Other Number

uniIdchar → any Unicode Modifier Letter or Non-Spacing Mark
idchar → small | large | digit | uniIdchar | '

varid → small {idchar} ⟨reservedid⟩
conid → large {idchar}

- GHC allows redundant parantheses around the function name in the `funlhs` part of declarations.
That is GHC will succeed in parsing a declaration like `((f)) x = ` for any number
of parantheses around `f`.

.. _infelicities-syntax:

Context-free syntax
^^^^^^^^^^^^^^^^^^^

- In Haskell 98 mode (but not in Haskell 2010 mode), GHC
is a little less strict about the layout rule when used in ``do``
expressions. Specifically, the restriction that "a nested context
must be indented further to the right than the enclosing context" is
relaxed to allow the nested context to be at the same level as the
enclosing context, if the enclosing context is a ``do`` expression.

For example, the following code is accepted by GHC: ::

main = do args <- getArgs
if null args then return [] else do
ps <- mapM process args
mapM print ps

This behaviour is controlled by the :extension:`NondecreasingIndentation`
extension.

.. extension:: NondecreasingIndentation
:shortdesc: Allow nested contexts to be at the same indentation level as
its enclosing context.

:since: 7.2.1

:status: Included in :extension:`Haskell98`

Allow nested contexts to be at the same indentation level as
its enclosing context.

- GHC doesn't do the fixity resolution in expressions during parsing as
required by Haskell 98 (but not by Haskell 2010). For example,
according to the Haskell 98 report, the following expression is
legal: ::

let x = 42 in x == 42 == True

and parses as: ::

(let x = 42 in x == 42) == True

because according to the report, the ``let`` expression “extends as
far to the right as possible”. Since it can't extend past the second
equals sign without causing a parse error (``==`` is non-fix), the
``let``\-expression must terminate there. GHC simply gobbles up the
whole expression, parsing like this: ::

(let x = 42 in x == 42 == True)

.. _infelicities-exprs-pats:

Expressions and patterns
^^^^^^^^^^^^^^^^^^^^^^^^

By default, GHC makes some programs slightly more defined than they should be.
For example, consider ::

f :: [a] -> b -> b
f [] = error "urk"
f (x:xs) = \v -> v

main = print (f [] `seq` True)

This should call ``error`` but actually prints ``True``. Reason: GHC
eta-expands ``f`` to

::

f :: [a] -> b -> b
f [] v = error "urk"
f (x:xs) v = v

For most programs this improves efficiency enough to be enabled
& bad only in few rare cases.
To suppress this optimisation use :ghc-flag:`-fpedantic-bottoms`.

.. _infelicities-failable-pats:

Failable patterns
^^^^^^^^^^^^^^^^^

Since the `MonadFail Proposal (MFP) `__,
do-notation blocks that contain a failable pattern need a `MonadFail `__ constraint.

For example

::

mayFail :: (MonadIO m) => m ()
mayFail = do
(Just value) <- fetchData
putStrLn value

Will warn you with

.. code-block:: none

• Could not deduce (MonadFail m)
arising from a do statement
with the failable pattern ‘(Just x)’
from the context: MonadIO m
bound by the type signature for:
mayFail :: forall (m :: * -> *). MonadIO m => m ()

And indeed, since the `Monad `__ class does not have the ``fail`` method anymore,
we need to explicitly add ``(MonadFail m)`` to the constraints of the function.

.. _infelicities-recursive-groups:

Typechecking of recursive binding groups
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The Haskell Report specifies that a group of bindings (at top level, or
in a ``let`` or ``where``) should be sorted into strongly-connected
components, and then type-checked in dependency order
(`Haskell Report, Section
4.5.1 `__). As
each group is type-checked, any binders of the group that have an
explicit type signature are put in the type environment with the
specified polymorphic type, and all others are monomorphic until the
group is generalised (`Haskell Report, Section
4.5.2 `__).

Following a suggestion of Mark Jones, in his paper `Typing Haskell in
Haskell `__, GHC implements a
more general scheme. In GHC *the dependency analysis ignores references to
variables that have an explicit type signature*. As a result of this refined
dependency analysis, the dependency groups are smaller, and more bindings will
typecheck. For example, consider: ::

f :: Eq a => a -> Bool
f x = (x == x) || g True || g "Yes"

g y = (y <= y) || f True

This is rejected by Haskell 98, but under Jones's scheme the definition
for ``g`` is typechecked first, separately from that for ``f``, because
the reference to ``f`` in ``g``\'s right hand side is ignored by the
dependency analysis. Then ``g``\'s type is generalised, to get ::

g :: Ord a => a -> Bool

Now, the definition for ``f`` is typechecked, with this type for ``g``
in the type environment.

The same refined dependency analysis also allows the type signatures of
mutually-recursive functions to have different contexts, something that is
illegal in Haskell 98 (Section 4.5.2, last sentence). GHC only insists that the
type signatures of a *refined* group have identical type signatures; in practice
this means that only variables bound by the same pattern binding must have the
same context. For example, this is fine: ::

f :: Eq a => a -> Bool
f x = (x == x) || g True

g :: Ord a => a -> Bool
g y = (y <= y) || f True

.. _infelicities-default-exports:

Default Module headers with -main-is
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The Haskell2010 Report specifies in that

"An abbreviated form of module, consisting only of the module body,
is permitted. If this is used, the header is assumed to be
`module Main(main) where`."

GHC's ``-main-is`` option can be used to change the name of the top-level entry
point from ``main`` to any other variable. When compiling the main module and
``-main-is`` has been used to rename the default entry point, GHC will also use
the alternate name in the default export list.

Consider the following program: ::

-- file: Main.hs
program :: IO ()
program = return ()

GHC will successfully compile this module with
``ghc -main-is Main.program Main.hs``, because the default export list
will include ``program`` rather than ``main``, as the Haskell Report
typically requires.

This change only applies to the main module. Other modules will still export
``main`` from a default export list, regardless of the ``-main-is`` flag.
This allows use of ``-main-is`` with existing modules that export ``main`` via
a default export list, even when ``-main-is`` points to a different entry
point, as in this example (compiled with ``-main-is MainWrapper.program``). ::

-- file MainWrapper.hs
module MainWrapper where
import Main

program :: IO ()
program = putStrLn "Redirecting..." >> main

-- file Main.hs
main :: IO ()
main = putStrLn "I am main."

.. _infelicities-Modules:

Module system and interface files
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

GHC requires the use of ``hs-boot`` files to cut the recursive loops
among mutually recursive modules as described in
:ref:`mutual-recursion`. This more of an infelicity than a bug: the
Haskell Report says (`Section
5.7 `__)

"Depending on the Haskell implementation used, separate compilation of
mutually recursive modules may require that imported modules contain
additional information so that they may be referenced before they are
compiled. Explicit type signatures for all exported values may be
necessary to deal with mutual recursion. The precise details of separate
compilation are not defined by this Report."

.. _infelicities-numbers:

Numbers, basic types, and built-in classes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``Num`` superclasses
The ``Num`` class does not have ``Show`` or ``Eq`` superclasses.

You can make code that works with both Haskell98/Haskell2010 and GHC
by:

- Whenever you make a ``Num`` instance of a type, also make
``Show`` and ``Eq`` instances, and

- Whenever you give a function, instance or class a ``Num t``
constraint, also give it ``Show t`` and ``Eq t`` constraints.

``Bits`` superclass
The ``Bits`` class does not have a ``Num`` superclass. It
therefore does not have default methods for the ``bit``, ``testBit``
and ``popCount`` methods.

You can make code that works with both Haskell 2010 and GHC by:

- Whenever you make a ``Bits`` instance of a type, also make a
``Num`` instance, and

- Whenever you give a function, instance or class a ``Bits t``
constraint, also give it a ``Num t`` constraint, and

- Always define the ``bit``, ``testBit`` and ``popCount`` methods
in ``Bits`` instances.

``Read`` class methods
The ``Read`` class has two extra methods, ``readPrec`` and
``readListPrec``, that are not found in the Haskell 2010 since they rely
on the ``ReadPrec`` data type, which requires the :extension:`RankNTypes`
extension. GHC also derives ``Read`` instances by implementing ``readPrec``
instead of ``readsPrec``, and relies on a default implementation of
``readsPrec`` that is defined in terms of ``readPrec``. GHC adds these two
extra methods simply because ``ReadPrec`` is more efficient than ``ReadS``
(the type on which ``readsPrec`` is based).

``Monad`` superclass
The ``Monad`` class has an ``Applicative`` superclass. You cannot write
``Monad`` instances that work for GHC and also for a Haskell 2010
implementation that does not define ``Applicative``.

Extra instances
The following extra instances are defined: ::

instance Functor ((->) r)
instance Monad ((->) r)
instance Functor ((,) a)
instance Functor (Either a)
instance Monad (Either e)

Multiply-defined array elements not checked
This code fragment should elicit a fatal error, but it does not: ::

main = print (array (1,1) [(1,2), (1,3)])

GHC's implementation of ``array`` takes the value of an array slot
from the last (index,value) pair in the list, and does no checking
for duplicates. The reason for this is efficiency, pure and simple.

.. _infelicities-Prelude:

In ``Prelude`` support
^^^^^^^^^^^^^^^^^^^^^^

``splitAt`` semantics
``Data.List.splitAt`` is more strict than specified in the Report.
Specifically, the Report specifies that ::

splitAt n xs = (take n xs, drop n xs)

which implies that ::

splitAt undefined undefined = (undefined, undefined)

but GHC's implementation is strict in its first argument, so ::

splitAt undefined [] = undefined

``Show``\ ing records
The Haskell 2010 definition of ``Show`` stipulates that the rendered
string should only include parentheses which are necessary to unambiguously
parse the result. For historical reasons, ``Show`` instances derived by GHC
include parentheses around records despite the fact that record syntax
binds more tightly than function application; e.g., ::

data Hello = Hello { aField :: Int } deriving (Show)

-- GHC produces...
show (Just (Hello {aField=42})) == "Just (Hello {aField=42})"

-- whereas Haskell 2010 calls for...
show (Just (Hello {aField=42})) == "Just Hello {aField=42}"

``Read``\ ing integers
GHC's implementation of the ``Read`` class for integral types
accepts hexadecimal and octal literals (the code in the Haskell 98
report doesn't). So, for example, ::

read "0xf00" :: Int

works in GHC.

A possible reason for this is that ``readLitChar`` accepts hex and
octal escapes, so it seems inconsistent not to do so for integers
too.

``isAlpha``
The Haskell 98 definition of ``isAlpha`` is: ::

isAlpha c = isUpper c || isLower c

GHC's implementation diverges from the Haskell 98 definition in the
sense that Unicode alphabetic characters which are neither upper nor
lower case will still be identified as alphabetic by ``isAlpha``.

``hGetContents``
Lazy I/O throws an exception if an error is encountered, in contrast
to the Haskell 98 spec which requires that errors are discarded (see
Section 21.2.2 of the Haskell 98 report). The exception thrown is
the usual IO exception that would be thrown if the failing IO
operation was performed in the IO monad, and can be caught by
``System.IO.Error.catch`` or ``Control.Exception.catch``.

.. _infelicities-ffi:

The Foreign Function Interface
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

``hs_init()``, ``hs_exit()``
The FFI spec requires the implementation to support re-initialising
itself after being shut down with ``hs_exit()``, but GHC does not
currently support that. See :ghc-ticket:`13693`.

.. index::
single: hs_init
single: hs_exit

.. _haskell-98-2010-undefined:

GHC's interpretation of undefined behaviour in Haskell 98 and Haskell 2010
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section documents GHC's take on various issues that are left
undefined or implementation specific in Haskell 98.

``Char``
.. index::
single: Char; size of

Following the ISO-10646 standard, ``maxBound :: Char`` in GHC is
``0x10FFFF``.

``Int``
.. index::
single: Int; size of
single: fromInteger function
single: fromIntegral function

In GHC the ``Int`` type follows the size of an address on the host
architecture; in other words it holds 32 bits on a 32-bit machine,
and 64-bits on a 64-bit machine.

Arithmetic on ``Int`` is unchecked for overflow\ ``Int``, so
all operations on ``Int`` happen modulo 2\ :sup:`⟨n⟩` where ⟨n⟩ is
the size in bits of the ``Int`` type.

The ``fromInteger`` (and hence also ``fromIntegral``) is a special case when
converting to ``Int``. The value of ``fromIntegral x :: Int`` is
given by taking the lower ⟨n⟩ bits of ``(abs x)``, multiplied by the
sign of ``x`` (in 2's complement ⟨n⟩-bit arithmetic). This behaviour
was chosen so that for example writing ``0xffffffff :: Int``
preserves the bit-pattern in the resulting ``Int``.

Negative literals, such as ``-3``, are specified by (a careful
reading of) the Haskell Report as meaning
``Prelude.negate (Prelude.fromInteger 3)``. So ``-2147483648`` means
``negate (fromInteger 2147483648)``. Since ``fromInteger`` takes the
lower 32 bits of the representation,
``fromInteger (2147483648::Integer)``, computed at type ``Int`` is
``-2147483648::Int``. The ``negate`` operation then overflows, but
it is unchecked, so ``negate (-2147483648::Int)`` is just
``-2147483648``. In short, one can write ``minBound::Int`` as a
literal with the expected meaning (but that is not in general
guaranteed).

The ``fromIntegral`` function also preserves bit-patterns when
converting between the sized integral types (``Int8``, ``Int16``,
``Int32``, ``Int64`` and the unsigned ``Word`` variants), see the
modules ``Data.Int`` and ``Data.Word`` in the library documentation.

Unchecked floating-point arithmetic
Operations on ``Float`` and ``Double`` numbers are *unchecked* for
overflow, underflow, and other sad occurrences. (note, however, that
some architectures trap floating-point overflow and
loss-of-precision and report a floating-point exception, probably
terminating the program)

.. index::
single: floating-point exceptions.

Large tuple support
The Haskell Report only requires implementations to provide tuple
types and their accompanying standard instances up to size 15. GHC
limits the size of tuple types to 62 and provides instances of
``Eq``, ``Ord``, ``Bounded``, ``Read``, ``Show``, and ``Ix`` for
tuples up to size 15.

.. _bugs:

Known bugs or infelicities
--------------------------

The bug tracker lists bugs that have been reported in GHC but not yet
fixed: see the `GHC issue tracker `__. In
addition to those, GHC also has the following known bugs or
infelicities. These bugs are more permanent; it is unlikely that any of
them will be fixed in the short term.

.. _bugs-ghc:

Bugs in GHC
~~~~~~~~~~~

- GHC's runtime system implements cooperative multitasking, with
context switching potentially occurring only when a program
allocates. This means that programs that do not allocate may never
context switch. This is especially true of programs using STM, which
may deadlock after observing inconsistent state. See :ghc-ticket:`367`
for further discussion.

If you are hit by this, you may want to compile the affected module
with :ghc-flag:`-fno-omit-yields <-fomit-yields>` (see :ref:`options-f`).
This flag ensures that yield points are inserted at every function entrypoint
(at the expense of a bit of performance).

- GHC does not allow you to have a data type with a context that
mentions type variables that are not data type parameters. For
example:

::

data C a b => T a = MkT a

so that ``MkT``\'s type is

::

MkT :: forall a b. C a b => a -> T a

In principle, with a suitable class declaration with a functional
dependency, it's possible that this type is not ambiguous; but GHC
nevertheless rejects it. The type variables mentioned in the context
of the data type declaration must be among the type parameters of the
data type.

- GHC's inliner can be persuaded into non-termination using the
standard way to encode recursion via a data type:

::

data U = MkU (U -> Bool)

russel :: U -> Bool
russel u@(MkU p) = not $ p u

x :: Bool
x = russel (MkU russel)

The non-termination is reported like this:

.. code-block:: none

ghc: panic! (the 'impossible' happened)
(GHC version 8.2.1 for x86_64-unknown-linux):
Simplifier ticks exhausted
When trying UnfoldingDone x_alB
To increase the limit, use -fsimpl-tick-factor=N (default 100)

with the panic being reported no matter how high a
:ghc-flag:`-fsimpl-tick-factor <-fsimpl-tick-factor=⟨n⟩>` you supply.

We have never found another class of programs, other than this
contrived one, that makes GHC diverge, and fixing the problem would
impose an extra overhead on every compilation. So the bug remains
un-fixed. There is more background in `Secrets of the GHC
inliner `__.

- On 32-bit x86 platforms when using the native code generator, the
:ghc-flag:`-fexcess-precision` option is always on.
This means that floating-point calculations are non-deterministic,
because depending on how the program is compiled (optimisation
settings, for example), certain calculations might be done at 80-bit
precision instead of the intended 32-bit or 64-bit precision.
Floating-point results may differ when optimisation is turned on. In
the worst case, referential transparency is violated, because for
example ``let x = E1 in E2`` can evaluate to a different value than
``E2[E1/x]``.

.. index::
single: -msse2 option

One workaround is to use the :ghc-flag:`-msse2` option (see
:ref:`options-platform`), which generates code to use the SSE2
instruction set instead of the x87 instruction set. SSE2 code uses
the correct precision for all floating-point operations, and so gives
deterministic results. However, note that this only works with
processors that support SSE2 (Intel Pentium 4 or AMD Athlon 64 and
later), which is why the option is not enabled by default. The
libraries that come with GHC are probably built without this option,
unless you built GHC yourself.

- The :ghc-flag:`state hack <-fno-state-hack>` optimization can result in
non-obvious changes in evaluation ordering which may hide exceptions, even
with :ghc-flag:`-fpedantic-bottoms` (see, e.g., :ghc-ticket:`7411`). For
instance, ::

import Control.Exception
import Control.DeepSeq
main = do
evaluate (('a' : undefined) `deepseq` return () :: IO ())
putStrLn "Hello"

Compiling this program with ``-O`` results in ``Hello`` to be printed,
despite the fact that ``evaluate`` should have bottomed. Compiling
with ``-O -fno-state-hack`` results in the exception one would expect.

- Programs compiled with :ghc-flag:`-fdefer-type-errors` may fail a bit
more eagerly than one might expect. For instance, ::

{-# OPTIONS_GHC -fdefer-type-errors #-}
main = do
putStrLn "Hi there."
putStrLn True

Will emit no output, despite the fact that the ill-typed term appears
after the well-typed ``putStrLn "Hi there."``. See :ghc-ticket:`11197`.

- Despite appearances ``*`` and ``Constraint`` aren't really distinct kinds
in the compiler's internal representation and can be unified producing
unexpected results. See :ghc-ticket:`11715` for one example.

- Because of a toolchain limitation we are unable to support full Unicode paths
on Windows. On Windows we support up to Latin-1. See :ghc-ticket:`12971` for more.

.. _bugs-ghci:

Bugs in GHCi (the interactive GHC)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- GHCi does not respect the ``default`` declaration in the module whose
scope you are in. Instead, for expressions typed at the command line,
you always get the default default-type behaviour; that is,
``default(Int,Double)``.

It would be better for GHCi to record what the default settings in
each module are, and use those of the 'current' module (whatever that
is).

- On Windows, there's a GNU ld/BFD bug whereby it emits bogus PE object
files that have more than 0xffff relocations. When GHCi tries to load
a package affected by this bug, you get an error message of the form

.. code-block:: none

Loading package javavm ... linking ... WARNING: Overflown relocation field (# relocs found: 30765)

The last time we looked, this bug still wasn't fixed in the BFD
codebase, and there wasn't any noticeable interest in fixing it when
we reported the bug back in 2001 or so.

The workaround is to split up the .o files that make up your package
into two or more .o's, along the lines of how the ``base`` package does
it.