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module Main where
import Control.Exception
import Data.Char
import System.IO
import GHC.Data.StringBuffer
twoBOMs = "T6016-twoBOMs"
ignoreFirstBOM = do
-- StringBuffer should not contain initial byte-order mark.
--
-- Just skipping over it, but leaving it in the Stringbuffer, is not
-- sufficient. The Lexer calls prevChar when a regular expression
-- starts with '^' (which is a shorthand for '\n^'). It would never
-- match on the first line, since instead of '\n', prevChar would
-- still return '\xfeff'.
s <- hGetStringBuffer twoBOMs
assert (prevChar s '\n' == '\n') return ()
dontIgnoreSecondBOM = do
-- U+FEFF is considered a BOM only if it appears as the first
-- character of a file.
h <- openBinaryFile twoBOMs ReadMode
hSeek h AbsoluteSeek 3
s <- hGetStringBufferBlock h 3
hClose h
assert (currentChar s == '\xfeff') return ()
main = do
writeFile twoBOMs "\xfeff\xfeff"
ignoreFirstBOM
dontIgnoreSecondBOM
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