GHC vs the Haskell 98 language Haskell 98 language vs GHC This section lists Glasgow Haskell infelicities in its implementation of Haskell 98. See also the “when things go wrong” section () for information about crashes, space leaks, and other undesirable phenomena. The limitations here are listed in Haskell Report order (roughly). The Haskell report specifies that programs may be written using Unicode. GHC only accepts the ISO-8859-1 character set at the moment. 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 .\. GHC doesn't do fixity resolution in expressions during parsing. For example, according to the Haskell report, the following expression is legal Haskell: 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) The Haskell report is arguably wrong here, but nevertheless it's a difference between GHC & Haskell 98. None known. None known. None known. 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)]) The Char type Charsize of The Haskell report says that the Char type holds 16 bits. GHC follows the ISO-10646 standard a little more closely: maxBound :: Char in GHC is 0x10FFFF. Arbitrary-sized tuples 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. Reading integers GHC's implementation of the Read class for integral types accepts hexadeciaml 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. This section documents GHC's take on various issues that are left undefined or implementation specific in Haskell 98. Sized integral types Intsize of 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 overflowoverflowInt , so all operations on Int happen modulo 2n where n is the size in bits of the Int type. The fromIntegerfromInteger function (and hence also fromIntegralfromIntegral ) 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 float 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)floating-point exceptions. In addition to the divergences from the Haskell 98 standard listed above, GHC has the following known bugs or infelicities. GHC can warn about non-exhaustive or overlapping patterns (see , and usually does so correctly. But not always. It gets confused by string patterns, and by guards, and can then emit bogus warnings. The entire overlap-check code needs an overhaul really. Dangers with multiple Main modules. GHC does not insist that module Main lives in a file called Main.hs. This is useful if you want multiple versions of Main. But there's a danger: when compiling module Main (regardless of what file it comes from), GHC looks for the interface Main.hi; it uses this to get version information from the last time it recompiled Main. The trouble is that this Main.hi may not correspond to the source file being compiled. Solution: remove Main.hi first. A better solution would be for GHC to record the source-file filename in the interface file, or even an MD5 checksum. 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). GHCi does not keep careful track of what instance declarations are 'in scope' if they come from other packages. Instead, all instance declarations that GHC has seen in other packages are all in scope everywhere, whether or not the module from that package is used by the command-line expression. 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) 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.