Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
- Loading branch information
Showing
1 changed file
with
59 additions
and
0 deletions.
There are no files selected for viewing
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -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 | ||
------------------------------------------------------------ |