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variables & values

val vs var

val maxHealth = 20 // immutable (can never be reassigned)
var currentHealth = 20 // mutable (can be reassigned)

trying to reassign a val is a runtime error:

val name = "Steve"
name = "Alex" // error: cannot reassign 'val name': it is immutable

use var for anything that changes like counters, flags, current game state, etc. use val for everything else, or don't, you do you

katton has exactly five kinds of runtime value, plus null:

type example notes
number 5, 3.5, -2 no separate int/float, everything is a decimal
string "hello" double quoted only
boolean true, false  
list ["a", "b", "c"] ordered and mutable
map { "key": "value" } insertion ordered and mutable
null null represents "nothing"

assignment operators

var score = 0
score = 10 // plain assignment
score += 5 // score = score + 5 -> 15
score -= 3 // score = score - 3 -> 12
score *= 2 // score = score * 2 -> 24
score /= 4 // score = score / 4 -> 6

top level vs local

var globalCounter = 0 // top level: shared across every function in this script

function increment() {
    var localTemp = 1 // local: only exists inside this call
    globalCounter += localTemp
}

null

val item = event.item   // might be null if there's no item involved

if (item != null) {
    // safe to use item here
}

always check for null before using a value that might not be present: event.item, Katton.pick() on an empty list, and Katton.vars.*.get(...) on an unset key can all return null

hi :D