Lacks Control.Lazy and our parser will be full-featured.

spago.dhall

@@ -12,7 +12,15 @@ to generate this file without the comments in this block.
 -}
 { name = "parser"
 , dependencies =
-  [ "arrays", "console", "effect", "maybe", "prelude", "strings", "tuples" ]
+  [ "arrays"
+  , "console"
+  , "control"
+  , "effect"
+  , "maybe"
+  , "prelude"
+  , "strings"
+  , "tuples"
+  ]
 , packages = ./packages.dhall
 , sources = [ "src/**/*.purs", "test/**/*.purs" ]
 }

src/Main.purs

@@ -39,10 +39,6 @@ main :: Effect Unit
 main = do
   log "🍝"
 
-  log $ "parsing 'hello': " <> case parse (stringP "hello") "hello" of
-    Just (Tuple x y) -> show x <> " " <> show y
-    Nothing -> "failed"
-
   log $ "parsing the first 'f' in 'fiction' (sat): " <> case parse (sat (\x -> x == 'f')) "fiction" of
     Just (Tuple x y) -> show x <> " " <> show y
     Nothing -> "failed"

src/Parser.purs

@@ -2,6 +2,11 @@ module Parser where
 
 import Prelude
 
+import Control.Alt (class Alt)
+import Control.Plus (class Plus, empty)
+import Control.Alternative (class Alternative)
+-- TODO: import Control.Lazy (class Lazy)
+
 import Data.Array as A
 import Data.Maybe (Maybe(..))
 import Data.Tuple (Tuple(..))
@@ -41,9 +46,25 @@ instance bindParser :: Bind Parser where
       let (Parser p2) = f x
       in p2 xs
 
--- Next steps.
+instance altParser :: Alt Parser where
-empty :: forall a. Parser a
+  alt :: forall a. Parser a -> Parser a -> Parser a
-empty = Parser \_ -> Nothing
+  alt (Parser p1) (Parser p2) = Parser p
+    where
+      p stream = case p1 stream of
+        Nothing -> p2 stream
+        Just x -> Just x
+
+instance plusParser :: Plus Parser where
+  empty :: forall a. Parser a
+  empty = Parser \_ -> Nothing
+
+instance alternativeParser :: Alternative Parser
+
+-- TODO: Control.Lazy instance
+--instance lazyParser :: Lazy Parser (Array a) where
+--  defer 
+
+-- Generic parsing functions.
 
 sat :: (Char -> Boolean) -> Parser Char
 sat p = do x <- item
@@ -73,6 +94,12 @@ isAlpha c = A.any (\f -> f c) [isLower, isUpper]
 letter :: Parser Char
 letter = sat isAlpha
 
+isAlphaNum :: Char -> Boolean
+isAlphaNum c = A.any (\f -> f c) [isAlpha, isDigit]
+
+alphanum :: Parser Char
+alphanum = sat isAlphaNum
+
 char :: Char -> Parser Char
 char x = sat (_ == x)
 
@@ -85,3 +112,18 @@ string str = case A.uncons (toCharArray str) of
   Just { head: x, tail: xs } -> do c <- char x
                                    rest <- string (fromCharArray xs)
                                    pure (concat c rest)
+
+-- TODO: A.many requires an instance of Control.Lazy class.
+--ident :: Parser String
+--ident = do x  <- lower
+--           xs <- A.many alphanum
+--           pure (fromCharArray $ A.cons x xs)
+
+-- TODO: try orElse
+--try :: Parser a -> Parser a
+--try p = Parser p'
+--  where
+--    p' = case parse p of
+--           Nothing -> p
+--           Just 
+-- orElse, (<|>) :: Parser a -> Parser a -> Parser a