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I have the below statements in Lex.x to parse block comments.

<0>  "//".*                 { tokWValue LTokComment   }            
<0>  "/*"                   { begin blockcomment      }            
<blockcomment> "*/"         { begin 0                 }            
<blockcomment> .            { tokWValue LTokComment   }

But If I generate Lex.hs using Alex, it does not add the 'begin' function. This results in the below compilation error. 

src/Lex.x:367:18: Not in scope: ‘begin’
src/Lex.x:368:18: Not in scope: ‘begin’

Any idea what might be wrong?

I am using wrapper 'posn'

dfordivam
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1 Answers1

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Start codes are only available when using any of the monad-... wrappers.

If you read the docs for the monad wrapper -- Section 5.3.3 - The "monad" wrapper -- you see that it is the first wrapper which keeps track of the start code.

You can also verify this by finding the alex wrapper files -- look for the directory containing the files AlexWrapper-basic, AlexWrapper-posn, etc. On OS X when installing the Haskell Platform they are located in a directory like /Library/Haskell/ghc-7.10.2-x86_64/share/alex-3.1.4. The functions begin and andBegin only occur in the monad-related wrappers.

ErikR
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    The Alex documentation does not mention any dependence on the wrappers. [alex documentation](https://www.haskell.org/alex/doc/html/alex-files.html). Anyways thanks for your reply! – dfordivam Nov 03 '15 at 07:58