Constant E0004_MUTATE_IMMUTABLE_VARIABLE

pub const E0004_MUTATE_IMMUTABLE_VARIABLE: ErrorCode;

E0004: Cannot reassign to a variable declared as immutable

Variables declared with let are immutable by default.

• ✅ Write let mut if you want to change their value after initialization.

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Example of erroneous code

let b = 4;
b = 5;

// autogenerated

Or

let a;
a = 2;
a = 3;

// autogenerated

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Explanation

In Compose, variables are immutable by default. This means you can assign to them once, but attempting to reassign will produce an error.

To allow a variable to change over time, declare it as mutable using mut:

let mut counter = 0;
counter = counter + 1;

This design prevents accidental changes and helps promote safer and more predictable code.

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Fixes

✅ Make the variable mutable:

let mut a = 2;
a = 3;

✅ Or, use shadowing (create a new variable):

let a = 2;
// ...
let a = 3; // this creates a new `a`

Shadowing can be a good alternative to mutation, especially in functional-style code.