9  Conformance with Haskell 98

9.1  Haskell 98 features not in Hugs

• Mutually recursive modules are not supported.
• Some library functions have been moved into the Prelude. This is necessary because the Prelude and the standard libraries, as defined in the Haskell 98 report, are mutually recursive. This mutual recursion has been avoided by moving the following functions into the Prelude:
  • From Ix: Ix(range, index, inRange, rangeSize).
  • From Char: isAscii, isControl, isPrint, isSpace, isUpper, isLower, isAlpha, isDigit, isOctDigit, isHexDigit, isAlphanum, digitToInt, intToDigit, toUpper, toLower, ord, and chr.
  • From Ratio: Ratio, Rational, (%), numerator, denominator, and approxRational.
• Derived Read instances do not work for some infix constructors. If an infix constructor has left associativity and the type appears recursively on the left side of the constructor, then the read instance will loop.
• Hugs does not allow the use of qualified names in instance declarations.
• Hugs does not use the Unicode character set yet. Characters are currently drawn from the ISO Latin-1 set.
• Two adjacent dashes -- start a one line comment; for strictly technical reasons, the change from Haskell 1.4 to Haskell 98 to use maximal munch for such comments has not yet been implemented.
• The floating point printer is not exactly as defined in the report. The printed form of a floating point number may re-read as a slightly different number.
• Derived instances for large tuples are not supplied. Instances for tuples larger than 5 (3 in the 16 bit PC system) are not in the Prelude.
• When using getArgs, only the stand-alone system passes arguments to the executing program. The interactive system always uses an empty argument list when runnning a program.
• The syntax of sections is slightly different. For example, the Haskell expression (2*3+) must instead be written as ((2*3)+).
• Instead of IO.hIsEOF, hugs provides IO.hugsHIsEOF. Whereas hIsEOF should tell you if the next call of hGetChar would raise an EOF error; hugsHIsEOF tells you if the last call of hGetCHar raised an EOF error (the same as ANSI C's feof).
• We ignore entity lists in qualified imports (but unqualified imports are treated correctly). For example, you can write:
      import qualified Prelude ( foo )
  even though foo is not exported from the Prelude and you can write:
    module M() where
    import qualified Prelude () -- import nothing
    x = Prelude.length "abcd"
  
• The Double type is implemented as a single precision float (this isn't forbidden by the standard but it is unusual).

9.2  Libraries

In the IO library, these functions are not defined: handlePosn, ReadWriteMode hFileSize, hIsEOF, isEOF, hSetBuffering, hGetBuffering, hSeek, hIsSeekable, hReady, and hLookahead. The following non-standard functions are exported:
  hugsGetCh :: IO Char   -- getchar without echoing to screen
  hugsHIsEOF :: Handle -> IO Bool
    -- same semantics as C's "feof" (different from Haskell's hIsEOF)
  hugsIsEOF  :: IO Bool
    -- same semantics as C's "feof(stdin)"
  hPutStrLn  :: String -> IO ()
    -- corresponds to Prelude.putStrLn

9.3  Haskell 98 extensions

• Import declarations may specify a file name instead of a module name.
• The T(..) syntax is allowed for type synonyms in import and export lists.