A recursive function "game" in Haskell

Question

I don't quite understand the following code:

game :: ([Move], Towers) -> Towers
game ([], towers) = towers
game situation = game (move situation)

situation here has never mentioned in any part of the codes (there are a long code before this, called the tower of Hanoi, I suppose a lot of people here know it). Why can we directly use situation here? I know this is correct and the code works very well, but I don't know why.

btw: I think this is the 3rd or 4th question on this in the last couple of days - is there a MOOC or something that has this as an exercise? — Random Dev
– Random Dev, Commented May 6, 2015 at 16:16
A rather trivial observation, move must extract elements from the list [Move] (perhaps according to some yet not all situations), in order to ensure the base case of empty list holds at some point, and thus ensure termination of the game. — elm
– elm, Commented May 6, 2015 at 16:18

bheklilr · Accepted Answer · 2015-05-06 15:44:23Z

4

situation is an argument to the function game. It would have the type ([Move], Towers). Essentially, what you're saying is "if situation has no moves then return the towers, otherwise perform a move and then pass that result to game".

It would be perfectly legal to write this function as

game ([], towers) = towers
game (moves, towers) = game (move (moves, towers))

But this requires taking apart a tuple then constructing a new one exactly like it, or you could use any other name for this value:

game ([], towers) = towers
game foobar = game (move foobar)

It's nothing more than a name for the argument to the function game, what it's actually called isn't important (so long as it's not a reserved keyword, of course, you couldn't name it import, for example).

answered May 6, 2015 at 15:44

bheklilr

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1 Comment

Benjamin Hodgson Over a year ago

@bheklilr "this requires taking apart a tuple then constructing a new one exactly like it" - Correct me if I'm wrong but this is something I would expect the compiler to optimise away.

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