7.11. 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) for information about crashes, space leaks, and other undesirable phenomena.

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

7.11.1. Divergence from Haskell 98

7.11.2. 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.

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 fromIntegerfunction (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).