Url Parsing in Idris

Here’s the translation of the Go URL parsing example to Idris:

Our URL parsing program demonstrates how to parse URLs in Idris. URLs provide a uniform way to locate resources.

import Data.String
import Data.List
import Data.Maybe

-- Define a record to hold URL components
record URL where
  constructor MkURL
  scheme : String
  user : Maybe (String, Maybe String)
  host : String
  port : Maybe String
  path : String
  query : String
  fragment : Maybe String

-- Parse a URL string into a URL record
parseURL : String -> Maybe URL
parseURL s = do
  let parts = split (== ':') s
  scheme <- head' parts
  rest <- tail' parts >>= head'
  let withoutScheme = dropWhile (== '/') rest
  let (authority, pathAndRest) = break (== '/') withoutScheme
  let (userInfo, hostPort) = break (== '@') authority
  let (host, maybePort) = break (== ':') (if hostPort == "" then userInfo else hostPort)
  let (path, queryAndFragment) = break (== '?') pathAndRest
  let (query, fragment) = break (== '#') (if queryAndFragment == "" then "" else tail queryAndFragment)
  
  pure $ MkURL scheme (parseUserInfo userInfo) host (tailMay maybePort) 
              ("/" ++ path) query (nullToNothing fragment)
  where
    parseUserInfo : String -> Maybe (String, Maybe String)
    parseUserInfo "" = Nothing
    parseUserInfo s = 
      let (user, pass) = break (== ':') s
      in Just (user, tailMay pass)
    
    tailMay : String -> Maybe String
    tailMay "" = Nothing
    tailMay s  = Just (tail s)
    
    nullToNothing : String -> Maybe String
    nullToNothing "" = Nothing
    nullToNothing s  = Just s

-- Main function to demonstrate URL parsing
main : IO ()
main = do
  let s = "postgres://user:pass@host.com:5432/path?k=v#f"
  case parseURL s of
    Nothing -> putStrLn "Failed to parse URL"
    Just u -> do
      putStrLn $ "Scheme: " ++ u.scheme
      putStrLn $ "User Info: " ++ show u.user
      putStrLn $ "Host: " ++ u.host
      putStrLn $ "Port: " ++ show u.port
      putStrLn $ "Path: " ++ u.path
      putStrLn $ "Query: " ++ u.query
      putStrLn $ "Fragment: " ++ show u.fragment

This Idris code defines a URL record to hold the components of a URL, and a parseURL function to parse a URL string into this record. The main function demonstrates how to use this parser.

To run the program:

$ idris url_parsing.idr -o url_parsing
$ ./url_parsing
Scheme: postgres
User Info: Just ("user", Just "pass")
Host: host.com
Port: Just "5432"
Path: /path
Query: k=v
Fragment: Just "f"

This output shows the different pieces extracted from the URL. Note that Idris, being a purely functional language, handles errors and optional values using Maybe types, which is different from Go’s approach of returning error values.

Also, Idris doesn’t have built-in networking libraries like Go’s net/url, so we’ve implemented a basic URL parser from scratch. In a real-world scenario, you might want to use a more robust parsing library or implement more comprehensive error handling and validation.

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Title here

Url Parsing in Idris