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This function decodes form-encoded content (e.g. "q=5&value=%20space%20")

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The string operator replace, finds an occurrence of its first argument, a regular expression, in the string and replaces it with the second argment string value. When the regular expression has the modifier g then all matching occurrences are replaced. According to a post in stackoverflow, "Encoders used the "+" character as a replacement for a space character in the early days", so we'll replace them first (line 2), and then do %20 (line 3). Actual plus signs would appear as %2B (or %2b).

The function hex2char could be written as:

    hex2char = function(encoded,hex) { // string len 2 -> character or encoded
      val = hex2dec(hex);
      hex.length() == 2 && val && val < 128
        => (val).chr()
         | encoded
    }

We get the decimal equivalent of the string passed in, which would be in hexadecimal, in line 2. Line 3 verifies that everything is valid, and returns a string with a single character (line 4), using the number operator chr. If something is wrong, we'll return instead, the value that we were asked to replace (line 5), in this case, %20, leaving the caller's string unchanged.

The function hex2dec could be written as:

    hex2dec = function(hex) { // string len k -> number in [0,16^k-1] or null
      hex like re#^[0-9A-Fa-f]*$#
        => hex.split("")
              .map(function(h){hexDigit2dec(h)})
              .reduce(function(a,h){a*16+h},0)
         | null
    }

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We return the value zero if we were given a character which is not a hexadecimal digit.

Ruleset

A ruleset with this code can be found at https://raw.githubusercontent.com/Picolab/pico_lessons/master/decode_content/edu.byu.picos.www.krl