Understanding Rust Error E0089: Wrong Number of Type Arguments

Introduction to Rust Error E0089

Rust Error E0089 occurs when you provide too few type arguments for a function. This error typically happens when the code expects multiple type arguments, but receives fewer than expected.

Breaking Down the Error Message

Let's take a closer look at the official documentation to understand the error message: Error code E0089 Note: this error code is no longer emitted by the compiler. Too few type arguments were supplied for a function. For example: fn foo<T, U>() {} fn main() { foo::<f64>(); // error: wrong number of type arguments: expected 2, found 1 } The example provided demonstrates a case where the function foo expects two type arguments (T and U), but only receives one (f64). This results in the error, stating that it expected 2 type arguments but found only 1.

Using the Correct Number of Type Arguments

To fix Rust Error E0089, you need to provide the correct number of type arguments for the function. To illustrate, let's adjust the previous example to use the required number of type arguments:

fn foo<T, U>() {}

fn main() {
    foo::<f64, bool>(); // No error: correct number of type arguments (2)
}

In this example, we provide both the required type arguments (f64 and bool), so the error is resolved.

Using Type Placeholders

In some cases, you may want the compiler to infer some of the type arguments for a function. To do this, you can use type placeholders (_), as shown in the official documentation: fn foo<T, U>(x: T) {} fn main() { let x: bool = true; foo::<f64>(x); // error: wrong number of type arguments: expected 2, found 1 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)` } In this example, the type placeholder (_) in the call to foo allows the compiler to infer the first type argument as bool.