Table of Contents
This section lists Glasgow Haskell infelicities in its implementation of Haskell 98 and Haskell 2010. See also the “when things go wrong” section (Chapter 10, What to do when something goes wrong) for information about crashes, space leaks, and other undesirable phenomena.
The limitations here are listed in Haskell Report order (roughly).
By default, GHC mainly aims to behave (mostly) like a Haskell 2010
compiler, although you can tell it to try to behave like a
particular version of the language with the
-XHaskell98 and
-XHaskell2010 flags. The known deviations
from the standards are described below. Unless otherwise stated,
the deviation applies in Haskell 98, Haskell 2010 and
the default modes.
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 .\.
In Haskell 98 mode and by default (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
NondecreasingIndentation extension.
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 == Trueand 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)The Haskell Report allows you to put a unary
- preceding certain expressions headed by
keywords, allowing constructs like - case x of
... or - do { ... }. GHC does
not allow this. Instead, unary - is
allowed before only expressions that could potentially
be applied as a function.
In its default mode, 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
This improves efficiency slightly but significantly for most programs, and
is bad for only a few. To suppress this bogus "optimisation" use -fpedantic-bottoms.
In its default mode, GHC does not accept datatype contexts,
as it has been decided to remove them from the next version of the
language standard. This behaviour can be controlled with the
DatatypeContexts extension.
See Section 7.4.2, “Data type contexts”.
GHC requires the use of hs-boot
files to cut the recursive loops among mutually recursive modules
as described in Section 4.7.9, “How to compile mutually recursive modules”. 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."
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.
The Bits class does not have
a Num superclasses. It therefore
does not have default methods for the
bit,
testBit and
popCount methods.
You can make code that works with both Haskell2010 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.
The following extra instances are defined:
instance Functor ((->) r) instance Monad ((->) r) instance Functor ((,) a) instance Functor (Either a) instance Monad (Either e)
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.
Tuples are currently limited to size 100. HOWEVER:
standard instances for tuples (Eq,
Ord, Bounded,
Ix Read, and
Show) are available
only up to 16-tuples.
This limitation is easily subvertible, so please ask if you get stuck on it.
splitAt semanticsReading integersGHC'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.
isAlphaThe 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.
This section documents GHC's take on various issues that are left undefined or implementation specific in Haskell 98.
Char type
Following the ISO-10646 standard,
maxBound :: Char in GHC is
0x10FFFF.
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, so all operations on Int happen
modulo
2n
where n is the size in bits of
the Int type.
The fromInteger function (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.
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).