Minecraft Datapack: Research on How to Implement Control Flow Structures

June 2, 2026 | Exploration to find a proper way to implement control flow structures inside Minecraft datapack.

I'm developing a language, where you write high-level code similar to Java, C, Python, etc, and compile the code into datapack.

This article is more of an exploration to find proper implementations to stably convert high-level code into datapack functions, which means the methods I discuss here is not for ordinary datapack development.

It's been a long time since my last writing without LLM. This article is wholy written by myself. It gives me a place to think really deeply while writing.

I will discuss how to implement these control flow structures in datapack:

• If
• While, as well as Continue and Break
• For

We know that computer programs use operations like jump or condition jump to build up control flow structures like if, while, etc. However, we don't have jump inside datapack. What we only have is /function and /return, which are the only two commands that can change the flow of control.

That means, if we want to implement control flow structures like if, while inside datapack, we have two options: one is to figure out how to simulate jump with minecraft commands, another is to implement them without jump.

I've found that it isn't intuitive to implement a complete version of control flow structures like If and While without an operation like jump. (See Why Do We Need Jump?)

However, there's a good news. I've figured out how to simulate jump inside minecraft datapack. Let's dive deep.

Simulate Jump

The only two things we can use are /function and /return. Say we have these two functions:

say 1
function minecraft:callee
say 3

say 2

If you run /function minecraft:caller, it's obvious we will get output like 1 2 3, right? What if we want to do a jump operation after say 1? That means we will only get output like 1 2 with say 3 skipped.

The method is to use return run function minecraft:callee instead of function minecraft:callee. Minecraft will first run say 1, then switch to function callee, which is say 2, then it goes back to function caller and directly return. All commands after it are actually skipped.

say 1
return run function minecraft:callee
say 3

say 2

The following code will have a similar effect but it's easier for you to determine return value:

say 1
function minecraft:callee
return 0 # any integers you want
say 3

say 2

Multiple jump will work well too.

say 1
return run function minecraft:chain1
say 2

say 3
return run function minecraft:chain2
say 4

say 5
return 1
say 6

Run /function minecraft:caller, we will get output 1 3 5, skipping say 3, say 4 and say 6. It's just the same as jump, right?

If

Single Branch If

Single Branch If is easy to implement. Say we have this high-level code:

say("before_if");
if(condition) {
    say("if_then");
}
say("after_if");

Represent it in mcfunction:

say before_if
execute unless <condition> run return run function minecraft:after_if
say if_then
return run function minecraft:after_if

say after_if

Two Branch If

say("before_if");
if(condition) {
    say("if_then");
} else {
    say("if_else");
}
say("after_if");

It's similar to Single Branch If. Before if_then, we just need to jump to if_else position instead of after_if position, then jump to after_if position inside if_else.

say before_if
execute unless <condition> run return run function minecraft:if_else
say if_then
return run function minecraft:after_if

say if_else
return run function minecraft:after_if

say after_if

Multiple Branch If

say("before_if");
if(condition1) {
    say("branch1");
} else if(condition2) {
    say("branch2");
} else {
    say("branch3");
}
say("after_if");

Try to jump to branch2 before branch1. And try to jump to branch3 before branch2. Start to perceive the regularity?

say before_if
execute unless <condition1> run return run function minecraft:branch2
say branch1
return run function minecraft:after_if

execute unless <condition1> run return run function minecraft:branch3
say branch2
return run function minecraft:after_if

say branch3
return run function minecraft:after_if

say after_if

Summary

The core principle is to do a jump to the next branch (or after_if if there's only one branch) at the beginning of branches except the last branch, when the branch's condition is not met.

• For Single Branch If, unless jump to after_if before if_then.
• For Two Branch If, unless jump to if_else before if_then.
• For Multiple Branch If, unless jump to branch2 before branch1, and unless jump to branch3 before branch2.

While

say("before_while");
while(condition) {
    say("a_round");
}
say("after_while");

To implement While, we jump to loop_body directly, and try to leave the loop_body at the beginning of it using a jump to after_while. At the end of loop_body we jump back to itself so it keeps looping.

say before_while
return run function minecraft:loop_body

execute unless <condition> run return run function minecraft:after_while
say a_round
return run function minecraft:loop_body

say after_while

For Loop

Since we've figured out how to implement While Loop, we can build a For Loop based on it.

Actually, For Loop can always be rewritten to a While Loop.

// For loop
for(i = 0;i <= 5;i++) {
    say(i);
}

// Rewrite to while loop
i = 0;
while(i <= 5) {
    say(i);
    i++;
}

You see it? To turn a For Loop into a While Loop, you only need to move the int i = 0 to the front of while and move the i++ to the last line of while.

Here is an example of mcfunction:

say before_while
# int i = 0;
scoreboard players set i var 0
return run function minecraft:loop_body

# while(i <= 5)
execute unless score i var matches ..5 run return run function minecraft:after_while
tellraw @a {"score": {"objective": "var", "name": "i"}}
# i++;
scoreboard players add i var 1
return run function minecraft:loop_body

say after_while

You can see that we just add several lines of code to the mcfunction code of While above.

Continue

This is actually the most tricky one if you don't find a way to simulate jump operation. That's why I think it's better to implement While with jump instead of without it.

With jump, we can directly jump to start a new round again(by jumping to loop_body). Say we want to output 0 to 5 but skip 3.

i = 0;
while(i <= 5) {
    if(i == 3) continue;
    say(i);
    i++;
}

Here is the mcfunction code. We'll jump to a new mcfunction increment, where the variable i increments and jump to start a new loop round.

say before_while
# int i = 0;
scoreboard players set i var 0
return run function minecraft:loop_body

# while(i <= 5)
execute unless score i var matches ..5 run return run function minecraft:after_while
# if(i == 3) continue;
execute if score i var matches 3 run return run function minecraft:increment
tellraw @a {"score": {"objective": "var", "name": "i"}}
# i++;
scoreboard players add i var 1
return run function minecraft:loop_body

# This is a new file!
# i++;
scoreboard players add i var 1
return run function minecraft:loop_body

say after_while

Why don't I just replace loop_body:line7-8 with return run function minecraft:increment? Because most of the time i can directly increment inside loop_body instead of running another /function, better performance.

Break

Compared to Continue, Break is far more easier to implement. You jump out of the loop directly, and it's done. Say we stop the loop when i equals to 3.

i = 0;
while(i <= 5) {
    if(i == 3) break;
    say(i);
    i++;
}

The mcfunction. Easy, right?

say before_while
# int i = 0;
scoreboard players set i var 0
return run function minecraft:loop_body

# while(i <= 5)
execute unless score i var matches ..5 run return run function minecraft:after_while
# if(i == 3) break;
execute if score i var matches 3 run return run function minecraft:after_while
tellraw @a {"score": {"objective": "var", "name": "i"}}
# i++;
scoreboard players add i var 1
return run function minecraft:loop_body

say after_while

Why Do We Need Jump?

All the things we talk about in this article are built on top of jump. That's because it will be far more easier to build them using jump.

There are a lot of differences between jump and /function command: /function will go back to where we run /function after the function completes, but jump won't come back itself. jump has a great advantage that you can determine yourself whether you want to go back to its caller because you can directly jump to the original position if you want. In comparison, /function must go back, no matter whether you want or not.