hw10: starting material

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+{- 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
+------------------------------------------------------------