Paul Graham refers to all those features as "orthogonality" ("On Lisp", pg. 63) and you're right, Haskell has it in spades, but it takes time to understand all of it and even more time to use it effectively. One almost needs a checklist.
But I think I'm catching on. I programmed this craps simulation last week. It's a problem from "Problems For Computer Solution", Gruenberger & Jaffray, 1965, The RAND Corp. import Control.Monad.State import System.Random type GeneratorState = State StdGen data Craps a = Roll a | Win a | Lose a deriving (Show) f :: Craps [Int] -> GeneratorState (Craps [Int]) f (Roll []) = do g0 <- get let (d1,g1) = randomR (1,6) g0 (d2,g2) = randomR (1,6) g1 t1 = d1+d2 put g2 case t1 of 2 -> return (Lose [t1]) 3 -> return (Lose [t1]) 7 -> return (Win [t1]) 11 -> return (Win [t1]) _ -> do g2 <- get let (d3,g3) = randomR (1,6) g2 (d4,g4) = randomR (1,6) g3 t2 = d3+d4 put g4 if t2 == t1 then do return (Win [t1,t2]) else if t2 == 7 then do return (Lose [t1,t2]) else f (Roll [t2,t1]) f (Roll l) = do g0 <- get let (d1,g1) = randomR (1,6) g0 (d2,g2) = randomR (1,6) g1 t = d1+d2 if t == (last l) then do put g2 return (Win (reverse (t:l))) else if t == 7 then do put g2 return (Lose (reverse (t:l))) else do put g2 f (Roll (t:l)) progressive (z@(x:xs),n) (Win _) = let b = x + (last xs) in (init xs,n+b) progressive (z@(x:xs),n) (Lose _) = let b = x + (last xs) in (z ++ [b],n-b) *Main> let r = evalState (sequence $ replicate 6 (f (Roll []))) (mkStdGen 987) *Main> r [Win [8,12,10,3,8],Win [5,9,10,11,12,11,8,9,5],Win [7],Lose [9,7],Win [5,5],Win [5,2,6,4,6,8,5]] *Main> foldl progressive ([1..10],0) r ([6],49) Function f generates the roll cycle outcomes which are then folded with the progressive betting system. In the final answer, the [6] is what's left of the original betting list [1..10]. The betting list is used to determine the bet: always bet the (first + last) of betting list. If a win, delete the first and last. If a loss, add loss to end of betting list. The 49 is winnings, initially 0. There's no explanation in the book of what should happen if the betting list becomes empty, or a singleton, but that could be fixed by making it longer. Comments, criticism, and better ways of doing it are welcome. Michael --- On Fri, 12/17/10, David Leimbach <leim...@gmail.com> wrote: From: David Leimbach <leim...@gmail.com> Subject: Re: [Haskell-cafe] Why is Haskell flagging this? To: "michael rice" <nowg...@yahoo.com> Cc: haskell-cafe@haskell.org, "Daniel Fischer" <daniel.is.fisc...@googlemail.com> Date: Friday, December 17, 2010, 7:45 PM No problem. Haskell is a different animal than even other functional languages in my experience, and it takes time to get used to the coolness in the type system, the lazy evaluation, the point free style, functional composition and all the other interesting techniques you now have at your fingertips for writing very expressive code :-). Do that for a while then go back to algol based languages, and wonder why the heck anyone uses those on purpose :-). (yeah there's good reasons to use them, but it starts to feel confining) Dave On Fri, Dec 17, 2010 at 4:28 PM, michael rice <nowg...@yahoo.com> wrote: Hi, all. Plenty of answers. Thank you. Putting the list in the IO monad was deliberate. Another one I was looking at was f :: String -> IO String f s = do return s main = do ios <- f "hello" fmap tail ios which worked fine So, the big error was trying to add 1 + [1,2,3,4,5]. I considered that I needed an additional fmap and thought I had tried fmap (fmap (1+)) iol but must have messed it up, because I got an error. I guess I was on the right track. I like to try various combinations to test my understanding. It's kind of embarrassing when I get stumped by something simple like this, but that's how one learns. Thanks again, Michael --- On Fri, 12/17/10, Daniel Fischer <daniel.is.fisc...@googlemail.com> wrote: From: Daniel Fischer <daniel.is.fisc...@googlemail.com> Subject: Re: [Haskell-cafe] Why is Haskell flagging this? To: haskell-cafe@haskell.org Cc: "michael rice" <nowg...@yahoo.com> Date: Friday, December 17, 2010, 4:24 PM On Friday 17 December 2010 18:04:20, michael rice wrote: > I don't understand this error message. Haskell appears not to understand > that 1 is a Num. > > Prelude> :t 1 > 1 :: (Num t) => t > Prelude> :t [1,2,3,4,5] > [1,2,3,4,5] :: (Num t) => [t] > Prelude> > > Michael > > =================== > > f :: [Int] -> IO [Int] > f lst = do return lst > > main = do let lst = f [1,2,3,4,5] > fmap (+1) lst The fmap is relative to IO, your code is equivalent to do let lst = (return [1,2,3,4,5]) fmap (+1) lst ~> fmap (+1) (return [1,2,3,4,5]) ~> do lst <- return [1,2,3,4,5] return $ (+1) lst but there's no instance Num [Int] in scope You probably meant do let lst = f [1,2,3,4,5] fmap (map (+1)) lst > > =============================== > > Prelude> :l test > [1 of 1] Compiling Main ( test.hs, interpreted ) > > test.hs:5:17: > No instance for (Num [Int]) > arising from the literal `1' at test.hs:5:17 > Possible fix: add an instance declaration for (Num [Int]) > In the second argument of `(+)', namely `1' > In the first argument of `fmap', namely `(+ 1)' > In the expression: fmap (+ 1) lst > Failed, modules loaded: none. > Prelude> --- On Fri, 12/17/10, Daniel Fischer <daniel.is.fisc...@googlemail.com> wrote: From: Daniel Fischer <daniel.is.fisc...@googlemail.com> Subject: Re: [Haskell-cafe] Why is Haskell flagging this? To: haskell-cafe@haskell.org Cc: "michael rice" <nowg...@yahoo.com> Date: Friday, December 17, 2010, 4:24 PM On Friday 17 December 2010 18:04:20, michael rice wrote: > I don't understand this error message. Haskell appears not to understand > that 1 is a Num. > > Prelude> :t 1 > 1 :: (Num t) => t > Prelude> :t [1,2,3,4,5] > [1,2,3,4,5] :: (Num t) => [t] > Prelude> > > Michael > > =================== > > f :: [Int] -> IO [Int] > f lst = do return lst > > main = do let lst = f [1,2,3,4,5] > fmap (+1) lst The fmap is relative to IO, your code is equivalent to do let lst = (return [1,2,3,4,5]) fmap (+1) lst ~> fmap (+1) (return [1,2,3,4,5]) ~> do lst <- return [1,2,3,4,5] return $ (+1) lst but there's no instance Num [Int] in scope You probably meant do let lst = f [1,2,3,4,5] fmap (map (+1)) lst > > =============================== > > Prelude> :l test > [1 of 1] Compiling Main ( test.hs, interpreted ) > > test.hs:5:17: > No instance for (Num [Int]) > arising from the literal `1' at test.hs:5:17 > Possible fix: add an instance declaration for (Num [Int]) > In the second argument of `(+)', namely `1' > In the first argument of `fmap', namely `(+ 1)' > In the expression: fmap (+ 1) lst > Failed, modules loaded: none. > Prelude> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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