[Haskell-cafe] Why is Haskell flagging this?
michael rice
nowgate at yahoo.com
Wed Dec 22 03:43:32 CET 2010
I changed your die function to rollDie in function roll2Dice (I assume that's what you meant) but get the errors listed below.
Michael
========================
import Control.Monad.State
import Control.Monad
import System.Random
type GeneratorState = State StdGen
genRandom :: Random a => GeneratorState a
genRandom = State random
-- similar
genRandomR :: Random a => (a,a) -> GeneratorState a
genRandomR = State . randomR
rollDie :: GeneratorState Int
rollDie = genRandomR (1,6)
roll2Dice :: GeneratorState Int
roll2Dice = liftM2 (+) rollDie rollDie
===========================
Prelude> :l craps7
[1 of 1] Compiling Main ( craps7.hs, interpreted )
craps7.hs:7:12: Not in scope: data constructor `State'
craps7.hs:11:13: Not in scope: data constructor `State'
Failed, modules loaded: none.
Prelude>
--- On Tue, 12/21/10, Ryan Ingram <ryani.spam at gmail.com> wrote:
From: Ryan Ingram <ryani.spam at gmail.com>
Subject: Re: [Haskell-cafe] Why is Haskell flagging this?
To: "michael rice" <nowgate at yahoo.com>
Cc: "David Leimbach" <leimy2k at gmail.com>, "Daniel Fischer" <daniel.is.fischer at googlemail.com>, haskell-cafe at haskell.org
Date: Tuesday, December 21, 2010, 7:00 PM
First, let's make some useful operations in your GeneratorState monad:
-- State :: (s -> (a,s)) -> State s a
-- random :: Random a => StdGen -> (a, StdGen)
genRandom :: Random a => GeneratorState a
genRandom = State random
-- similar
genRandomR :: Random a => (a,a) -> GeneratorState a
genRandomR = State . randomR
rollDie :: GeneratorState Int
rollDie = genRandomR (1,6)
roll2Dice :: GeneratorState Int
roll2Dice = liftM2 (+) die die
These can be used to simplify a lot of the code here.
-- ryan
On Fri, Dec 17, 2010 at 5:55 PM, michael rice <nowgate at yahoo.com> wrote:
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 <leimy2k at gmail.com> wrote:
From: David Leimbach <leimy2k at gmail.com>
Subject: Re: [Haskell-cafe] Why is Haskell flagging this?
To: "michael rice" <nowgate at yahoo.com>
Cc: haskell-cafe at haskell.org, "Daniel Fischer" <daniel.is.fischer at 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 <nowgate at 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.fischer at googlemail.com> wrote:
From: Daniel Fischer <daniel.is.fischer at googlemail.com>
Subject: Re: [Haskell-cafe] Why is Haskell flagging this?
To: haskell-cafe at haskell.org
Cc: "michael rice" <nowgate at 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.fischer at googlemail.com> wrote:
From: Daniel Fischer <daniel.is.fischer at googlemail.com>
Subject: Re: [Haskell-cafe] Why is Haskell flagging this?
To: haskell-cafe at haskell.org
Cc: "michael rice" <nowgate at 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 at haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe at haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20101221/ce702478/attachment.htm>
More information about the Haskell-Cafe
mailing list