[Haskell-beginners] Problem with minimax with alpha beta pruning

[Maxim Frolov]

Got it. Thank you Kim-Ee!
On 22 Dec 2020, 08:28 +0000, Kim-Ee Yeoh <ky3 at atamo.com>, wrote:
> Hi Maxim,
>
> The scope of this question falls outside this beginners list, which tends toward questions about haskell syntax (see other active thread).
>
> You will typically find more response on the haskell-cafe list, which you might want to resend your query to.
>
> > On Mon, Dec 21, 2020 at 11:01 PM Maxim Frolov <maxim.frolov.07 at gmail.com> wrote:
> > > Hi All,
> > >
> > > I am new to Haskell and got stuck while experimenting with minimax algorithm (tic-tac-toe game). Just for learning I am trying to avoid external modules and use only the standard Prelude library.
> > >
> > > Here is the code snippet:
> > >
> > > type Pos = (Int,Int)
> > > -- Players are O (minimizing) and X (maximizing), B is for the draw (blank), I and J are used for -INF and +INF respectively
> > > data Player = I | O | B | X | J
> > >     deriving (Eq, Ord, Show)
> > > type Grid = [(Pos,Player)]
> > > data Tree a = Node a [Tree a]
> > >     deriving Show
> > >
> > > minimax :: Player -> Player -> Tree Grid -> Tree (Grid, Player)
> > > minimax _ _ (Node g [])
> > >  | wins X g = Node (g,X) []
> > >  | wins O g = Node (g,O) []
> > >  | otherwise = Node (g,B) []
> > > minimax a b (Node g ts)
> > >  | turn g == X =
> > >    let ts' = [minimax alpha b t | t <- ts, alpha < b]
> > >        ps = [p | Node (_,p) _ <- ts']
> > >        alpha = maximum (a:ps)
> > >    in Node (g, alpha) ts'
> > >  | turn g == O =
> > >    let ts' = [minimax a beta t | t <- ts, a < beta]
> > >        ps = [p | Node (_,p) _ <- ts']
> > >        beta = minimum (b:ps)
> > >    in Node (g, beta) ts'
> > >
> > >
> > > The function call is like:
> > >
> > > minimax I J tree
> > >
> > >
> > > It looks like I got a recursion loop. Could someone advise how to approach the problem?
> > >
> > > Thank you,
> > > Max
> > >
> > >
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> --
> -- Kim-Ee
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