#PARSEC HASKELL CODE#

ThatвЂ™s it! To use these rules, weвЂ™ll add a little main function: We donвЂ™t care to keep the quotes, so weвЂ™ll just extract and return whatвЂ™s between them:

Parsec canвЂ™t use just plain вЂњ|вЂќ because thatвЂ™s a reserved token in Haskell, used in pattern matching. In this case, weвЂ™re going to ignore the optional whitespace, and just return the field-value, so: This rule is going to return a list of whatever field returns.ДЇield =, field-value, I could have invoked the sepBy function with prefix notation ( sepBy field (char вЂ,вЂ™)), which would have saved me the back-ticks, but in that case the word вЂњByвЂќ is followed by вЂњfieldвЂќ, which doesnвЂ™t read right to me. This will parse fields, separated by a single comma. This rule fits a common case in parsing of elements separated by other elements. ThatвЂ™s a literal translation, but Parsec provides a function that will make this a lot simpler. Then we return the first field followed by the remainder. So, we do a case on what we got and construct the remainder of the field list as either another field_list or an empty list, respectively. The optionMaybe function returns a Maybe type, which is either Just the syntactic element weвЂ™re looking for, or Nothing if that element isnвЂ™t provided. We obtain the first field, then look for an optional comma. We can state that rule in Haskell as:Д®asy, huh? The many function is provided by Parsec, and means exactly the same thing as the braces in Extended Backus-Naur Form (EBNF). Which indicates that a csv-file is composed of many rows. DonвЂ™t worry, Haskell functions are often one-liners. Now, weвЂ™ll proceed to convert each line of the CSV BNF we presented earlier into a Haskell function. In order to make use of this module, we have to import it: Parsec is a parsing module that comes distributed with the Glasgow Haskell Compiler (GHC). To work this bit oвЂ™ magic, we first need a proper spellbook.

#PARSEC HASKELL CODE#

Today weвЂ™ll see just how easy it is to translate those rules into Haskell code that will parse a CSV file. In a previous post, I provided a general overview of Backus-Naur Form (BNF) and an example set of BNF rules for comma-separated values (CSVs). Parsing CSV by feeding BNF to HaskellвЂ™s Parsec moduleД¬hip Camden shows how easy it is to translate Backus-Naur Form rules into Haskell code that will parse a comma-separated file.

Parsec haskell