Redundancy

The Flix compiler aggressively rejects programs that contain unused elements. The idea is to help programmers avoid subtle bugs[^1]. While this can take some time getting used to, we believe the trade-off is worth it.

Specifically, the Flix compiler will ensure that a program does not have:

• Unused local variables: local variables that are declared, but never used.
• Shadowed local variables: local variables that shadow other local variables.
• Useless expressions: pure expressions whose values are discarded.

Unused Local Variables

Flix rejects programs with unused variables.

For example, the following program is rejected:

def main(): Unit \ IO =
    let x = 123;
    let y = 456;
    println("The sum is ${x + x}")

with the message:

❌ -- Redundancy Error -------------------------------------------------- Main.flix

>> Unused local variable 'y'. The variable is not referenced within its scope.

3 |     let y = 456;
            ^
            unused local variable.

Unused local variables can be prefixed by an underscore _ to suppress the error. For example, if we replace y by _y the above program compiles:

def main(): Unit \ IO =
    let x = 123;
    let _y = 456; // OK
    println("The sum is ${x + x}")

Shadowed Local Variables

Flix rejects programs with shadowed variables.

For example, the following program is rejected:

def main(): Unit \ IO =
    let x = 123;
    let x = 456;
    println("The value of x is ${x}.")

with the message:

❌ -- Redundancy Error -------------------------------------------------- Main.flix

>> Shadowed variable 'x'.

3 |     let x = 456;
            ^
            shadowing variable.

The shadowed variable was declared here:

2 |     let x = 123;
            ^
            shadowed variable.

Useless Expressions

Flix rejects programs with pure expressions whose results are discarded.

For example, the following program is rejected:

def main(): Unit \ IO =
    123 + 456;
    println("Hello World!")

with the message:

❌ -- Redundancy Error -------------------------------------------------- Main.flix

>> Useless expression: It has no side-effect(s) and its result is discarded.

2 |     123 + 456;
        ^^^^^^^^^
        useless expression.

The expression has type 'Int32'

An expression that has no side-effect and whose result is unused is suspicious, since it could just be removed from the program without changing its meaning.

[^1] See e.g. Using Redundancies to Find Errors.