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Today's Topics:
1. parse block comments (mike h)
2. Re: parse block comments (Francesco Ariis)
3. Re: parse block comments (mike h)
4. Re: parse block comments (mike h)
5. State, StateT, or ST for game settings? (Dave Martin)
6. Re: State, StateT, or ST for game settings? (Francesco Ariis)
----------------------------------------------------------------------
Message: 1
Date: Sat, 18 Mar 2017 21:11:20 +0000
From: mike h <mike_k_hough...@yahoo.co.uk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] parse block comments
Message-ID: <c3fadf0f-a3f8-4338-a4d1-9fb1101fe...@yahoo.co.uk>
Content-Type: text/plain; charset=utf-8
Hi,
Below is code I’m building up for simple monadic and applicative parsers from
first principles.
I’ve just added code for single line Haskell style comments.
-- comment being '--'
comment :: Parser ()
comment = do
string "--"
many (sat ( /='\n') )
return ()
— or as applicative
comment' = string "--" *> many (sat (/='\n') )
I’d really appreciate help on writing a function for multi line comments! “{-“
and end with “-}”
I know I need some sort of look ahead but can’t quite get the feel of how!
Thanks
Mike
======================================
import Control.Applicative
import Data.Char
newtype Parser a = P (String -> [(a,String)])
parse :: Parser a -> String -> [(a,String)]
parse (P p) = p
item :: Parser Char
item = P (\s -> case s of
[] -> []
(x:xs) -> [(x, xs)])
instance Functor Parser where
-- fmap :: (a -> b) -> f a -> f b
fmap g p = P (\inp -> case parse p inp of
[] -> []
[(x, xs)] -> [(g x, xs)])
instance Applicative Parser where
-- pure :: a -> Parser a
-- (<*>) :: Parser (a -> b) -> Parser a -> Parser b
pure x = P (\inp -> [(x, inp)])
pab <*> pa = P (\inp -> case parse pab inp of
[] -> []
[(aTob, out)] -> parse (fmap aTob pa) out)
three :: Parser (Char, Char)
three = pure g <*> item <*> item <*> item
where g x y z = (x, z)
instance Monad Parser where
-- return :: a -> Parser a
return x = P (\inp -> [(x, inp)])
-- (>>=) :: Parser a -> (a -> Parser b) -> Parser b
pa >>= f = P (\inp -> case parse pa inp of
[] -> []
[(a, out)] -> parse (f a) out)
instance Alternative Parser where
-- empty :: Parser a
empty = P (\inp -> [])
-- (<|>) :: Parser a-> Parser a -> Parser a
-- if p1 works the that one otherwise p2
p1 <|> p2 = P (\inp -> case parse p1 inp of
[] -> parse p2 inp
out -> out)
sat :: (Char -> Bool) -> Parser Char
sat p = do
x <- item
if p x then return x else empty
digit :: Parser Char
digit = sat isDigit
lower :: Parser Char
lower = sat isLower
upper :: Parser Char
upper = sat isUpper
letter :: Parser Char
letter = sat isAlpha
alphanum :: Parser Char
alphanum = sat isAlphaNum
char :: Char -> Parser Char
char x = sat (==x)
string :: String -> Parser String
string [] = return []
string s@(x:xs) = do
char x *> string xs *> return s
ident :: Parser String
ident = do
x <- lower
xs <- many alphanum
return (x:xs)
ident' :: Parser String
ident' = pure (:) <*> lower <*> many alphanum
nat :: Parser Int
nat = do
xs <- some digit
return (read xs)
nat' :: Parser Int
nat' = pure read <*> some digit
space :: Parser ()
space = do
many (sat isSpace )
return ()
space' :: Parser ()
space' = many (sat isSpace ) *> return ()
int :: Parser Int
int = do
char '-'
n <- nat
return (-n)
<|> nat
int' :: Parser Int
int' = char '-' *> pure ((-1)*) <*> nat <|> nat
token :: Parser a -> Parser a
token p = do
space
v <- p
space
return v
token' :: Parser a -> Parser a
token' p = space *> p <* space
identifier :: Parser String
identifier = token ident
natural :: Parser Int
natural = token nat
integer :: Parser Int
integer = token int
symbol :: String -> Parser String
symbol xs = token (string xs)
nats :: Parser [Int]
nats = do
symbol "["
n <- natural
ns <- many ( do symbol ","
natural )
symbol "]"
return (n:ns)
-- comment being '--'
comment :: Parser ()
comment = do
string "--"
many (sat ( /='\n') )
return ()
comment' = string "--" *> many (sat (/='\n') )
nats' :: Parser [Int]
-- sequence and ignore "[" and the rest
-- the rest is fmap list cons into result of natural and the apply that to many
others and
-- finally ignore the closing "]"
nats' = symbol "[" >> (:) <$> natural <*> many (symbol "," >> natural) <*
symbol "]"
------------------------------
Message: 2
Date: Sat, 18 Mar 2017 22:47:15 +0100
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] parse block comments
Message-ID: <20170318214715.ga14...@casa.casa>
Content-Type: text/plain; charset=utf-8
On Sat, Mar 18, 2017 at 09:11:20PM +0000, mike h wrote:
> Hi,
>
> Below is code I’m building up for simple monadic and applicative parsers
> from first principles.
Hello Mike,
You might want to check `manyTill` from Parsec to get an idea:
manyTill :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m end ->
ParsecT s u m [a]
manyTill p end = scan
where
scan = do { end; return [] } <|>
do { x <- p; xs <- scan; return (x:xs) }
The 'trick' lies in <|> (alternative). Does that help?
------------------------------
Message: 3
Date: Sat, 18 Mar 2017 22:22:25 +0000
From: mike h <mike_k_hough...@yahoo.co.uk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] parse block comments
Message-ID: <7f565336-cb41-40ab-8ac8-250fc5efc...@yahoo.co.uk>
Content-Type: text/plain; charset="utf-8"
Hi Francesco,
:)
Yes! That really did help.
Will post later when I’ve tidied what I’ve done but it seems correct.
Thank you.
M
> On 18 Mar 2017, at 21:47, Francesco Ariis <fa...@ariis.it> wrote:
>
> On Sat, Mar 18, 2017 at 09:11:20PM +0000, mike h wrote:
>> Hi,
>>
>> Below is code I’m building up for simple monadic and applicative parsers
>> from first principles.
>
> Hello Mike,
> You might want to check `manyTill` from Parsec to get an idea:
>
> manyTill :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m end ->
> ParsecT s u m [a]
> manyTill p end = scan
> where
> scan = do { end; return [] } <|>
> do { x <- p; xs <- scan; return (x:xs) }
>
>
> The 'trick' lies in <|> (alternative). Does that help?
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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Message: 4
Date: Sat, 18 Mar 2017 22:49:11 +0000
From: mike h <mike_k_hough...@yahoo.co.uk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] parse block comments
Message-ID: <ab704f77-ce0b-4ec5-b0fa-a1b357f60...@yahoo.co.uk>
Content-Type: text/plain; charset=utf-8
Monadic and applicative.
blockCmnt :: Parser ()
blockCmnt = do
string "{-"
manyTill item (string "-}")
return ()
manyTill p endp = scan where
scan = do
endp
return []
<|>
do
x <- p
xs <- scan
return (x:xs)
blockCmnt' :: Parser ()
blockCmnt' = string "{-" >> manyTill item (string "-}") >> return ()
manyTill' p endp = scan' where
scan' = endp *> return [] <|> pure (:) <*> p <*> scan’
Thanks.
M
> On 18 Mar 2017, at 21:47, Francesco Ariis <fa...@ariis.it> wrote:
>
> On Sat, Mar 18, 2017 at 09:11:20PM +0000, mike h wrote:
>> Hi,
>>
>> Below is code I’m building up for simple monadic and applicative parsers
>> from first principles.
>
> Hello Mike,
> You might want to check `manyTill` from Parsec to get an idea:
>
> manyTill :: (Stream s m t) => ParsecT s u m a -> ParsecT s u m end ->
> ParsecT s u m [a]
> manyTill p end = scan
> where
> scan = do { end; return [] } <|>
> do { x <- p; xs <- scan; return (x:xs) }
>
>
> The 'trick' lies in <|> (alternative). Does that help?
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
------------------------------
Message: 5
Date: Sat, 18 Mar 2017 20:42:38 -0400
From: Dave Martin <davemartin...@aol.com>
To: beginners@haskell.org
Subject: [Haskell-beginners] State, StateT, or ST for game settings?
Message-ID: <15ae4041ade-58a-8...@webprd-a58.mail.aol.com>
Content-Type: text/plain; charset="utf-8"
I would like to create a game with a "main menu" offering the user a choice of
"playing the game" or accessing a "settings menu," in which he can change
defaults that would affect the game play (e.g. board size, etc.).
I know (barely) enough about the "IO monad" to write the menus and the basic
gameplay, but what device do I need in order to manage the settings? A State
monad? A StateT monad transformer? An ST monad? None of the above? I have some
basic familiarity with the first two, but not enough to really grasp how they
are actually used.
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------------------------------
Message: 6
Date: Sun, 19 Mar 2017 01:59:05 +0100
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] State, StateT, or ST for game
settings?
Message-ID: <20170319005905.ga3...@casa.casa>
Content-Type: text/plain; charset=us-ascii
On Sat, Mar 18, 2017 at 08:42:38PM -0400, Dave Martin wrote:
> I would like to create a game with a "main menu" offering the user a choice
> of "playing the game" or accessing a "settings menu," in which he can change
> defaults that would affect the game play (e.g. board size, etc.).
>
> I know (barely) enough about the "IO monad" to write the menus and the basic
> gameplay, but what device do I need in order to manage the settings? A State
> monad? A StateT monad transformer? An ST monad? None of the above? I have
> some basic familiarity with the first two, but not enough to really grasp how
> they are actually used.
Hello Dave,
State will do! Of course if you need to mix the two monads (State +
IO) you need a State transformer (StateT)
-F
------------------------------
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