Monday, November 2, 2020

Parser Combinator for SQL Subset

Some more records from RC... a fellow Recurser and I presented on our toy SQL parser combinator at the Friday 5-minute talks (talk slides).

We actually ended up writing parser combinators from scratch twice during RC (first for CIS194 and again for fp-course). The two had almost identical primitives like satisfy :: (Char -> Bool) -> Parser Char, zeroOrMore, oneOrMore, and building up from there... which makes me wonder if there's just a standard recipe for parser combinators? 

For the toy SQL parser, we ended up referencing how Stephen Diehl uses Parsec in Kaleidescope once we defined the syntax. It turns our that Parsec provides a bunch of primitives we previously wrote ourselves, like brackets, parens, commaSep... But I think it would have taken us much longer to figure out how to use the package, were it not for Diehl's examples.

*Database.Parser> process "Insert Into myTable [1, True, \"Jonathan\"];"

Insert "myTable" [FvInt 1,FvBool True,FvText "Jonathan"]

*Database.Parser> process "Insert Into myTable [1, True, \"Jonathan\"]"

(line 1, column 42):

unexpected end of input

expecting ";"

*Database.Parser> process "Insert myTable [1, True];"

(line 1, column 8):

unexpected "m"

expecting "Into"