From: Ramakrishnan Muthukrishnan Date: Fri, 26 Dec 2014 16:47:45 +0000 (+0530) Subject: hw10: starting material X-Git-Url: https://git.rkrishnan.org/about.html?a=commitdiff_plain;h=f1784118fd8f129b0a3290521d152dd93e3015bc;p=yorgey.git hw10: starting material --- diff --git a/hw10/10-applicative.pdf b/hw10/10-applicative.pdf new file mode 100644 index 0000000..d59188a Binary files /dev/null and b/hw10/10-applicative.pdf differ diff --git a/hw10/AParser.hs b/hw10/AParser.hs new file mode 100644 index 0000000..0ee3090 --- /dev/null +++ b/hw10/AParser.hs @@ -0,0 +1,59 @@ +{- CIS 194 HW 10 + due Monday, 1 April +-} + +module AParser where + +import Control.Applicative + +import Data.Char + +-- A parser for a value of type a is a function which takes a String +-- represnting the input to be parsed, and succeeds or fails; if it +-- succeeds, it returns the parsed value along with the remainder of +-- the input. +newtype Parser a = Parser { runParser :: String -> Maybe (a, String) } + +-- For example, 'satisfy' takes a predicate on Char, and constructs a +-- parser which succeeds only if it sees a Char that satisfies the +-- predicate (which it then returns). If it encounters a Char that +-- does not satisfy the predicate (or an empty input), it fails. +satisfy :: (Char -> Bool) -> Parser Char +satisfy p = Parser f + where + f [] = Nothing -- fail on the empty input + f (x:xs) -- check if x satisfies the predicate + -- if so, return x along with the remainder + -- of the input (that is, xs) + | p x = Just (x, xs) + | otherwise = Nothing -- otherwise, fail + +-- Using satisfy, we can define the parser 'char c' which expects to +-- see exactly the character c, and fails otherwise. +char :: Char -> Parser Char +char c = satisfy (== c) + +{- For example: + +*Parser> runParser (satisfy isUpper) "ABC" +Just ('A',"BC") +*Parser> runParser (satisfy isUpper) "abc" +Nothing +*Parser> runParser (char 'x') "xyz" +Just ('x',"yz") + +-} + +-- For convenience, we've also provided a parser for positive +-- integers. +posInt :: Parser Integer +posInt = Parser f + where + f xs + | null ns = Nothing + | otherwise = Just (read ns, rest) + where (ns, rest) = span isDigit xs + +------------------------------------------------------------ +-- Your code goes below here +------------------------------------------------------------