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Reploid

Member

From: Cold Country up North

Registered: 2008-03-27

Posts: 110

rly n00b help, haskell patterns

Hi!

Have a q on pattern matching.

How do I change all strings in a file that look like this:

StringWords, RestOfString

into:

NewStringWords, RestOfString

I have this textfile:

table=training
values=year, month, date, day, type, duration
2010, July, 9th, Friday, MTB, 160

Now, I want to convert this textfile, and similar textfiles into this:

insert into table=training(year, month, date, day, type, duration) values ('2010', 'July', '9th', 'Friday', 'MTB', '160');

Greatful for input.

EDIT: massive simplification of question.

Last edited by Reploid (2010-07-09 16:04:12)

Kazuki

Member

From: Germany

Registered: 2009-05-12

Posts: 15

Re: rly n00b help, haskell patterns

Based on chapter 4 of "Real World Haskell", section "Code Reuse Through Composition":

-- Convert the string into a list of strings and then foldr over it
-- to convert it row for row. After the fold, convert the list of strings back
-- into a string.
convertTableTraining :: String -> String
convertTableTraining = unwords . foldr convertRows [] . lines
  where convertRows x xs
            | "table="  `isPrefixOf` x =
                insertIntoTable x xs
            | "values=" `isPrefixOf` x =
                getValueNames x : xs
            | otherwise                =
                getValues x : xs

-- This is just there to have no whitespace between "training" and
-- the left parenthesis. If this isn't crucial, you can just omit
-- this function and cons "insert into table=training" to the list
-- above.
insertIntoTable :: String -> [String] -> [String]
insertIntoTable _ xs = ("insert into table=training" ++ head xs) : tail xs

-- Add the parentheses and drop "values=".
getValueNames :: String -> String
getValueNames xs = "(" ++ valueNames xs ++ ")"
  where valueNames = drop 7

-- Add "values ( )" and add "\n" at the end so that there is an
-- end-of-line.
getValues :: String -> String
getValues xs = "values (" ++ values xs ++ ");\n"
-- Filter the commas out since they get in the way after the quotes
-- have been added. This assumes that there is always a whitespace
-- after a comma. If you want to be safe, you could use the 'replace'
-- function in Data.String.Utils of the MissingH library. (Didn't try
-- it)
  where values xs = let filteredWords = words $ filter (/= ',') xs
                    in  unwords $ foldr addQuotes [] filteredWords
        addQuotes x [] = ("'" ++ x ++ "'" ) : []
        addQuotes x xs = ("'" ++ x ++ "',") : xs

So you basically read your file and use such a function on the input. This input gets converted into a list of strings and is being processed with a fold. After that the list of strings needs to be converted back into a single string which you can write to whatever output file you want.

Note that I'm not really experienced myself. I'm pretty sure there are better ways to accomplish this.