A HERE Document syntax

The conventional HERE document operator -- `<<` -- is not a good fit for Haskell. It's a perfectly legal user-level operator. I'd like to propose the use of backticks for HERE documents. Examples -------------------------------------------------------------- Double backtick would introduce an "indented HERE document", one that would be bounded by the layout rules in the same way as `do` blocks. Just like a `do`, the doubled backticks must be followed by whitespace. For example: rawXML = `` <a> <b> text </b> </a> This is just the same as the Haskell string: "<a>

<b> text </b>

</a>" Note that the final newline -- and any final whitespace -- is stripped. Non-indented HERE documents are also of value; these require a terminator as well as an initializer. Any number of backticks greater than 2 introduces a non-indented HERE document; the same number of backticks terminates it. All leading whitespace up to and including the first newline is consumed; all final whitespace is likewise consumed. For example: usage = ```` USAGE: hello_world <name>? Says hello, optionally to <name>. ```` This yields the Haskell string: "USAGE: hello_world <name>?



Says hello, optionally to <name>." There is a natural relationship between quasi-quoting and HERE documents. Thankfully, our rule about spaces gives us a way to integrate quasi-quoters. import Regexen(r) matchUTCDate = ``r` \d{4}-\d\d-\d\dT\d\d:\d\d:\d\dZ matchSource = ``r` [a-z]+\[\d+\] We use two backticks to introduce an indented HERE document and then specify the regular expression quasi-quote interpreter. This is brief and clear and not too full of demarcating cruft; it's not the usual `/[a-z]+\[\d+\]/` but it's still pretty nice. String Escapes & The Backslash Plague -------------------------------------------------------------- Naturally, one desires to use character escapes in HERE documents; however, the backslash plague is a bother and we'd not like that so much, either. Many Haskell escapes should be ignored. The string escapes for double quote and newline are unnecessary; likewise, we don't need to escape backslash. On the other hand, we can not do without character escapes for `\BEL`, Chinese characters and so forth. Let us adopt the rule that the 2 and 3 letter escapes are kept, as well as the hexadecimal escapes; the "control-with-character" escapes, octal escapes, decimal escapes and all single character escapes save `\&` are ignored. What if we want to document the escape rules of HERE documents in a HERE document? We have to have a way to "unescape" the escapes. The rule proposed is odd but simple: escaping = `` There comes a time when one desires to escape a magical character such as ASCII bell (\BEL). How shall we do it? We insert it with \BEL\&\\. \BEL\ We can insert a string of \BELs as long as they are terminated with with a \. We enter \BEL\BEL\BEL\&\\ to ding three times. \BEL\BEL\BEL\ As long as an escape pattern does not end with a backslash, it is interpreted literally. The empty string escape (\&) has an unusual power -- it is a "combo breaker" and prevents an otherwise legitimate sequence of escapes from being escaped. How do we output the literal we used for \BEL\&\\? It's \BEL \BS\\& \BS\\\. This example rings once when the first paragraph is printed and thrice when the second is printed. It becomes the Haskell string: unlines ["There comes a time when one desires to escape a magical" ,"character such as ASCII bell (\\BEL). How shall we do it?" ,"We insert it with \\BEL\\. \BEL" ,"" ,"We can insert a string of \\BELs as long as they are" ,"terminated with with a \\. We enter \\BEL\\BEL\\BEL\\ to" ,"ding three times. \BEL\BEL\BEL" ,"" ,"As long as an escape pattern does not end with a" ,"backslash, it is interpreted literally. The empty string" ,"escape (\\&) has an unusual power -- it is a \"combo" ,"breaker\" and prevents an otherwise legitimate sequence" ,"of escapes from being escaped. How do we output the" ,"literal we used for \\BEL\\? It's \\BEL \BS\\& \BS\\\\." ] This is the most sensible way that I can see to get rid of the need to escape backslashes but I grant it's a bit burdensome. I expect such advanced escaping to be rarely used. HERE documents are an aid to quasi-quotation and scripting. The syntax here proposed is simple, using only backticks in an unusual pattern, and allows for following the layout rule when that is desired. -- Jason Dusek