Re: erreta, a couple of unimportant missing words :-(
Ahn Ki-yung wrote:
Simple Cat (revisitied)
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { v - x; if f v then return (Just v) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h
\end{code}
This is my answer for the question of my own,
which is posted a couple
of days before.
There are mapM, filterM in the Haskell 98 Standard Library.
But why no findM there ?
As you can see from simple cat, it seems quite useful.
I think fildM should be added to the module Monad.
--
Ahn Ki-yung
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Why no findM ? simple Cat revisited
Simple Cat (revisitied)
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { v - x; if f v then return (Just v) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h
\end{code}
This is my answer for the question of my own,
which is posted a couple
There are mapM, filterM in the Haskell 98 Standard Library.
But why no findM there ?
As you can see from simple cat, it seems quite useful.
I think fildM should be added to the module Monad.
--
Ahn Ki-yung
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Re: Why no findM ? simple Cat revisited
Simple Cat (revisitied)
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { v - x; if f v then return (Just v) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h
\end{code}
Seems to me like the name findM could be misleading
mapM :: (Monad m) = (a - m b) - [a] - m [b]
filterM :: (Monad m) = (a - m Bool) - [a] - m [a]
These take a monadic function and a list of elements. Yours works the other
way around (takes a function and a list of 'monadic elements').
I'd expect the definition of findM to be:
findM' :: (Monad m) = (a - m Bool) - [a] - m (Maybe a)
findM' f [] = return Nothing
findM' f (x:xs) = do { b - f x; if b then return (Just x) else findM' f xs }
This one doesn't serve your purpose though.
J.A.
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Re: Why no findM ? simple Cat revisited
Jorge Adriano wrote:
Seems to me like the name findM could be misleading
mapM :: (Monad m) = (a - m b) - [a] - m [b]
filterM :: (Monad m) = (a - m Bool) - [a] - m [a]
These take a monadic function and a list of elements. Yours works the other
way around (takes a function and a list of 'monadic elements').
I'd expect the definition of findM to be:
findM' :: (Monad m) = (a - m Bool) - [a] - m (Maybe a)
findM' f [] = return Nothing
findM' f (x:xs) = do { b - f x; if b then return (Just x) else findM' f xs }
This one doesn't serve your purpose though.
J.A.
I appreciate your comment.
I agree that the type of findM should be the one you suggested,
and it still fits my original purpose. It's no more than a step arout.
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { b - f x; if b then return (Just x) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (=return.isLeft) (hCat stdin)
where hCat h = try (hGetLine h=putStrLn) : hCat h
\end{code}
I expetct the next Haskell Library Report includes findM.
It's obviously useful.
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simple cat by joining two infinite lists (intput/ouput)
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { b - f x; if b then return (Just x) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main =
findM (=return.isLeft) $
map (try . uncurry (=)) $
zip (hGetCharS stdin) (hPutCharS stdout)
where
hGetCharS h = hGetChar h : hGetCharS h
hPutCharS h = hPutChar h : hPutCharS h
\end{code}
Joining input list and output list by uncurried =
IO errors such as EOF are enclosed by try.
findM finds those EOF or IO errors.
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library of monadic functions [was: Why no findM ? simple Cat revisited]
I appreciate your comment.
I agree that the type of findM should be the one you suggested,
and it still fits my original purpose. It's no more than a step arout.
\begin{code}
import IO
findM f [] = return Nothing
findM f (x:xs) = do { b - f x; if b then return (Just x) else findM f xs }
isLeft (Left _) = True
isLeft _ = False
main = findM (=return.isLeft) (hCat stdin)
where hCat h = try (hGetLine h=putStrLn) : hCat h
\end{code}
Yes, you are right.
It was enough because, you don't really care about what you found, you just
want to search and stop when you do find something. You are returning the
action that returned an element that satisfied your
condition, not the actual element like before.
I expetct the next Haskell Library Report includes findM.
It's obviously useful.
I think both versions can be very useful:
findM :: (Monad m) = (a - m Bool) - [a] - m (Maybe a)
findM' :: (Monad m) = (a - Bool) - [m a] - m (Maybe a)
Same can be said for,
takeWhileM :: (Monad m) = (a - m Bool) - [a] - m [a]
takeWhileM' :: (Monad m) = (a - Bool) - [m a] - m [a]
both would be usefull for different purposes.
Oh and since we're on it I also miss,
iterateM :: (Monad m) = (a - m a) - a - m [a]
untilM :: (Monad m) = (a - m a) - a - m [a]
etc etc...
I've just been coding them as I need them, like many of you I suppose.
J.A.
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Re: library of monadic functions [was: Why no findM ? simple Cat revisited]
G'day all. On Wed, Nov 20, 2002 at 08:25:46PM +, Jorge Adriano wrote: I think both versions can be very useful: findM :: (Monad m) = (a - m Bool) - [a] - m (Maybe a) findM' :: (Monad m) = (a - Bool) - [m a] - m (Maybe a) I can also make a case for: findM'' :: (Monad m) = (a - Bool) - [m a] - m a findM'' p [] = fail findM'': not found findM'' p (x:xs) = p x = \b - if b then return x else findM'' p xs This goes with the philosophy that library functions shouldn't just return Maybe. Somewhere, somehow, there is a most general version of findM to be found. :-) Cheers, Andrew Bromage ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Random Color
I am trying to construct an infinate list of pairs of random colors. I am hung up on getting a random color. I have: data Color = Blue | Red | Green deriving (Eq, Ord, Show) am I supposed to instantiate a Random class instance from color? I am not sure exactly how the random number generator works. Is this the best way to go about it? Should I instead get a random Integer index of an array of Colors? thanks, -mike ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Random Color
G'day all. On Wed, Nov 20, 2002 at 08:44:36PM -0500, Mike T. Machenry wrote: I am trying to construct an infinate list of pairs of random colors. I am hung up on getting a random color. I have: data Color = Blue | Red | Green deriving (Eq, Ord, Show) am I supposed to instantiate a Random class instance from color? You could derive instances of Enum (and possibly also Bounded) and create random elements that way. Cheers, Andrew Bromage ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Random Color
Andrew and list, I am a beginer. I really don't know what I would do if I derived Color from Enum. You say I could create elements that way. Is there some simple example someone could post to the list? Thank you for your help. -mike On Thu, Nov 21, 2002 at 01:55:55PM +1100, Andrew J Bromage wrote: G'day all. On Wed, Nov 20, 2002 at 08:44:36PM -0500, Mike T. Machenry wrote: I am trying to construct an infinate list of pairs of random colors. I am hung up on getting a random color. I have: data Color = Blue | Red | Green deriving (Eq, Ord, Show) am I supposed to instantiate a Random class instance from color? You could derive instances of Enum (and possibly also Bounded) and create random elements that way. Cheers, Andrew Bromage ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
