From: Ramakrishnan Muthukrishnan <ram@rkrishnan.org>
Date: Sat, 27 Dec 2014 15:23:26 +0000 (+0530)
Subject: hw11: starting files
X-Git-Url: https://git.rkrishnan.org/pf/content/%22news.html/about.html?a=commitdiff_plain;h=456f59f9d65871460c92e8ef32c6936d2177993b;p=yorgey.git

hw11: starting files
---

diff --git a/hw11/11-applicative2.pdf b/hw11/11-applicative2.pdf
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diff --git a/hw11/AParser.hs b/hw11/AParser.hs
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+++ b/hw11/AParser.hs
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+module AParser (Parser, runParser, satisfy, char, posInt) where
+
+import           Control.Applicative
+import           Data.Char
+
+newtype Parser a = Parser { runParser :: String -> Maybe (a, String) }
+
+satisfy :: (Char -> Bool) -> Parser Char
+satisfy p = Parser f
+  where
+    f [] = Nothing
+    f (x:xs)
+        | p x       = Just (x, xs)
+        | otherwise = Nothing
+
+char :: Char -> Parser Char
+char c = satisfy (== c)
+
+posInt :: Parser Integer
+posInt = Parser f
+  where
+    f xs
+      | null ns   = Nothing
+      | otherwise = Just (read ns, rest)
+      where (ns, rest) = span isDigit xs
+
+inParser f = Parser . f . runParser
+
+first :: (a -> b) -> (a,c) -> (b,c)
+first f (x,y) = (f x, y)
+
+instance Functor Parser where
+  fmap = inParser . fmap . fmap . first
+
+instance Applicative Parser where
+  pure a = Parser (\s -> Just (a, s))
+  (Parser fp) <*> xp = Parser $ \s ->
+    case fp s of
+      Nothing     -> Nothing
+      Just (f,s') -> runParser (f <$> xp) s'
+
+instance Alternative Parser where
+  empty = Parser (const Nothing)
+  Parser p1 <|> Parser p2 = Parser $ liftA2 (<|>) p1 p2
diff --git a/hw11/SExpr.hs b/hw11/SExpr.hs
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+{- CIS 194 HW 11
+   due Monday, 8 April
+-}
+
+module SExpr where
+
+import AParser
+import Control.Applicative
+
+------------------------------------------------------------
+--  1. Parsing repetitions
+------------------------------------------------------------
+
+zeroOrMore :: Parser a -> Parser [a]
+zeroOrMore p = undefined
+
+oneOrMore :: Parser a -> Parser [a]
+oneOrMore p = undefined
+
+------------------------------------------------------------
+--  2. Utilities
+------------------------------------------------------------
+
+spaces :: Parser String
+spaces = undefined
+
+ident :: Parser String
+ident = undefined
+
+------------------------------------------------------------
+--  3. Parsing S-expressions
+------------------------------------------------------------
+
+-- An "identifier" is represented as just a String; however, only
+-- those Strings consisting of a letter followed by any number of
+-- letters and digits are valid identifiers.
+type Ident = String
+
+-- An "atom" is either an integer value or an identifier.
+data Atom = N Integer | I Ident
+  deriving Show
+
+-- An S-expression is either an atom, or a list of S-expressions.
+data SExpr = A Atom
+           | Comb [SExpr]
+  deriving Show