if statement in do block gives error message

Question

I'm trying to make a very simple snake-like game where, if you try to go to a x,y coordinate you have already visited, you lose the game.

This is code that's working so far (you can move player 1 with arrowkeys and player 2 with wasd):

import UI.NCurses

main :: IO ()    
main = runCurses $ do
    w <- defaultWindow
    updateWindow w $ do
        drawBorder Nothing Nothing Nothing Nothing Nothing Nothing Nothing Nothing
    render
    loop w [] 1 1 0 0 10 10 0 0

loop :: Window -> [(Integer, Integer)] -> Integer -> Integer -> Integer -> Integer -> Integer -> Integer -> Integer -> Integer ->  Curses ()
loop w list p1x p1y p1oldDx p1oldDy p2x p2y p2oldDx p2oldDy = do
    e <- getEvent w (Just 100)

    let coordList = updateXY e p1oldDx p1oldDy p2oldDx p2oldDy
    let p1dX = coordList !! 0
    let p1dY = coordList !! 1
    let p2dX = coordList !! 2
    let p2dY = coordList !! 3

    updateWindow w $ do
        moveCursor (p1y+p1dY) (p1x+p1dX)
        drawString ("#")
        moveCursor (p2y+p2dY) (p2x+p2dX)
        drawString ("#")
    render

    let updatedList = list ++ [(p1y+p1dY, p1x+p1dX)] ++ [(p2y+p2dY, p2x+p2dX)]

    loop w updatedList (p1x+p1dX) (p1y+p1dY) p1dX p1dY (p2x+p2dX) (p2y+p2dY) p2dX p2dY

updateXY :: Maybe Event -> Integer -> Integer  -> Integer -> Integer -> [Integer]
updateXY e p1oldX p1oldY p2oldX p2oldY
    | e == Just (EventSpecialKey KeyLeftArrow) = [-1, 0, p2oldX, p2oldY]
    | e == Just (EventSpecialKey KeyRightArrow) = [1, 0, p2oldX, p2oldY]
    | e == Just (EventSpecialKey KeyDownArrow) = [0, 1, p2oldX, p2oldY]
    | e == Just (EventSpecialKey KeyUpArrow) = [0, -1, p2oldX, p2oldY]
    | e == Just (EventCharacter 'a') = [p1oldX, p1oldY, -1, 0]
    | e == Just (EventCharacter 'd') = [p1oldX, p1oldY, 1, 0]
    | e == Just (EventCharacter 's') = [p1oldX, p1oldY, 0, 1]
    | e == Just (EventCharacter 'w') = [p1oldX, p1oldY, 0, -1]
    | p1oldX /= 0 = [p1oldX, 0, p2oldX, p2oldY]
    | p1oldY /= 0 = [0, p1oldY, p2oldX, p2oldY]
    | p2oldX /= 0 = [p1oldX, p1oldY, p2oldX, 0]
    | p2oldY /= 0 = [p1oldX, p1oldY, 0, p2oldY]
    | otherwise = [1, 0, 1, 0] -- Starts moving in x-direction. Change to (0,0) for stand-still

So as you move, "#" characters are put down. The above code does nothing if you go to a coordinate which already has a "#" on it, so I tried changing the loop function by adding this just before the let updatedList...:

if length (filter (==(p1x, p1y)) list) > 0 then gameOver w "player one lost"
if length (filter (==(p2x, p2y)) list) > 0 then gameOver w "player two lost"

And adding a temporary gameOver function:

gameOver w player = 
    updateWindow w $ do
        moveCursor (10) (10)
        drawString (player)
    render

But when I try to load this file in GHCI I get the following error message: parse error on input 'if'

`do` is not a function; it's a keyword that introduces a block of syntactic sugar. — chepner, May 20 '19 at 15:54

score 6 · Accepted Answer · answered May 20 '19 at 15:52

6

ifs need elses. Period. You can use the when function for when you need the imperative style "if this, then do that, else do nothing," but there is no built-in syntax:

import Control.Monad

when (length (filter (==(p1x, p1y)) list) > 0) $ gameOver w "player one lost"
when (length (filter (==(p2x, p2y)) list) > 0) $ gameOver w "player two lost"

The definition of when is

when cond action = if cond then action else pure ()

answered May 20 '19 at 15:52

HTNW

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but the loop will continue, won't it? (unless `gameOver` throws an exception, but that would be bad style). explicit `if` will force us to place `loop ...` into just one, appropriate clause. – Will Ness May 20 '19 at 15:58

score 4 · Answer 2 · answered May 20 '19 at 15:51

4

In Haskell, if expressions must have both then and else clauses.

Without the else, you get the error.

Both the consequent and the alternative clause must have the same type.

answered May 20 '19 at 15:51

Will Ness

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if statement in do block gives error message

2 Answers2