How to specify tab width for Alex lexer?
Alex documentation (Chapter 5) says:
You might want Alex to keep track of the line and column number in the input text, or you might wish to do it yourself (perhaps you use a different tab width from the standard 8-columns, for example)
But changing tab width from 8 to 4 in Alex position tracker is rather hard than easy. The code for this is hidden deep inside Alex generated routines:
-- this function is used by `alexGetByte`, which is used by `alex_scan_tkn`, which is
-- used by `alexScanUser` and `alexRightContext`,
-- which is used by `alex_accept` etc etc...
alexMove :: AlexPosn -> Char -> AlexPosn
alexMove (AlexPn a l c) 't' = AlexPn (a+1) l (((c+7) `div` 8)*8+1)
alexMove (AlexPn a l c) 'n' = AlexPn (a+1) (l+1) 1
alexMove (AlexPn a l c) _ = AlexPn (a+1) l (c+1)
One idea is to create your own wrapper which defines alexMove the way you want it.
On my Mac wrappers are installed in /Library/Haskell/ghc-7.6.3/lib/alex-3.0.5/share/
Look for where files named "AlexWrapper-monad", "AlexWrapper-monad-bytestring", ... reside on your system.
The "-t" command line option tells alex where to look for templates, but it also might pertain to wrappers since it appears that wrappers and templates reside in the same directory.
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