Changes between Version 4 and Version 5 of Commentary/Parser

Timestamp:

08/18/09 00:01:35 (4 years ago)

Author:

erikd (IP: 150.101.204.108)

Comment:

--

Commentary/Parser

@@ -17 +17 @@
   
 Furthermore, try-less parsing should not make the parser any more difficult to modify or extend. Quite the contrary, the try-less parser is easier debug and easier to extend once the concepts of try-less parsing are understood.
+
+As an example, a part of the expression parser (src/Source/Parser/Exp.hs) used to look like this
+
+{{{
+  <|>   -- \ case { ALT .. }
+        -- overlaps with the nexta lambda form
+        (Parsec.try $ do
+                tok     <- pTok K.BackSlash
+                pTok K.Case
+                alts    <- pCParen (Parsec.sepEndBy1 pCaseAlt pSemis)
+                return  $ XLambdaCase (spTP tok) alts)
+
+  <|>   -- \ PAT .. -> EXP
+        do      tok1    <- pTok K.BackSlash
+                pats    <- Parsec.many1 pPat1
+                pTok    <- pTok K.RightArrow
+                exp     <- pExp
+                return  $ XLambdaPats (spTP tok1) pats exp
+
+}}}
+
+Both of these combinators start with a K.Backslash token, with the first being wrapped in a Parsec.try. In the case of the first one failing, the second combinator, also starting with a K.Backslash token will be tried.
+
+Removing the Parsec.try in this instance involves replacing the above two combinators with:
+
+{{{
+  <|>   do      tok     <- pTok K.BackSlash
+                exp     <- pBackslashExp (spTP tok)
+                return  $ exp
+}}}
+
+which uses a new combinator pBackslashExp defined as:
+
+{{{
+pBackslashExp :: SP -> Parser (Exp SP)
+pBackslashExp startPos =
+        do      pTok K.Case
+                alts    <- pCParen (Parsec.sepEndBy1 pCaseAlt pSemis)
+                return  $ XLambdaCase startPos alts
+
+  <|>   do      pats    <- Parsec.many1 pPat1
+                pTok    <- pTok K.RightArrow
+                exp     <- pExp
+                return  $ XLambdaPats startPos pats exp
+
+}}}
+
+Notice that the call to pBackslashExp passes in the position of the K.Backslash token to be used in the return value.
+