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).
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 on the left hand side of a binding before deciding which symbol is the function symbol. For example, the following fails, because GHC makes the assumption that the function symbol is |-:
infix 5 |- infix 9 := data Equal = Char := Int 0 |- x:=y = 1 |- x:=y -- XXX fails here
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
(let x = 42 in x == 42) == True
(let x = 42 in x == 42 == True)
May not go through. If you add a “string gap” every few thousand characters, then the strings can be as long as you like.
Bear in mind that string gaps and the -cpp option don't mix very well (see Section 4.12.3).
It might work, but it's just begging for trouble.
Several modules internal to GHC are visible in the standard namespace. All of these modules begin with Prel, so the rule is: don't use any modules beginning with Prel in your program, or you will be comprehensively screwed.
Arguably not an infelicity, but… Bear in mind that operations on Int, 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).
Use Integer, Rational, etc., numeric types if this stuff keeps you awake at night.
This code fragment should elicit a fatal error, but it does not:
main = print (array (1,1) [(1,2), (1,3)])
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.
Tuples are currently limited to size 61. HOWEVER: standard instances for tuples (Eq, Ord, Bounded, Ix Read, and Show) are available only up to 5-tuples.
These limitations are easily subvertible, so please ask if you get stuck on them.