# Known bugs and infelicities

## Haskell 98 vs. Glasgow Haskell: language non-compliance

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).

### Divergence from Haskell 98

#### Lexical syntax

- 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 `.\`.

#### Context-free syntax

- 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

- 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.

#### Expressions and patterns

None known.

#### Declarations and bindings

GHC's typechecker makes all pattern bindings monomorphic by default; this behaviour can be disabled with `-fno-mono-pat-binds`. See .

#### Module system and interface files

None known.

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

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.

#### In `Prelude` support

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.

`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`.

### GHC's interpretation of undefined behaviour in Haskell 98

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

The `Char` type

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

Sized integral types

  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.

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)
  .

## Known bugs or infelicities

The bug tracker lists bugs that have been reported in GHC but not yet fixed: see the SourceForge GHC page. 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 in GHC

- 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.

- 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)

  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.

- GHC 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. This bug affects only the `--make` mode and GHCi.

- Separate compilation of modules when using the 'records with existential types' extension (see ) is broken. The only workaround is to always clean and build with `--make`, or to use a GHC 6.7 development snapshot. See trac bug #933 for more details.

### 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

      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.