Chapter 12. Known bugs and infelicities
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## Chapter 12. Known bugs and infelicities

## 12.1. 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 (Chapter 9, 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).

### 12.1.1. Divergence from Haskell 98

#### 12.1.1.1. 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 `.\`.

#### 12.1.1.2. 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.

#### 12.1.1.3. Expressions and patterns

None known.

#### 12.1.1.4. Declarations and bindings

GHC's typechecker makes all pattern bindings monomorphic by default; this behaviour can be disabled with `-fno-mono-pat-binds`. See Section 7.1, “Language options”.

#### 12.1.1.5. Module system and interface files

None known.

#### 12.1.1.6. 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.

#### 12.1.1.7. 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`.

### 12.1.2. 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 2`n` 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).

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