How do I specify that a function takes a HashMap?

Question

How do I specify a non-primitive type as a Rust function parameter - specifically, a HashMap? For example:

use std::collections::HashMap;

// a main function that would call fibbonacci...

// Here the hashmap would be used for memoizing;
// maybe ugly, but it's a first attempt
fn fibbonacci(n: i32, cache: ??) -> i32 {
}

I've tried:

• cache: Hashmap => wrong number of type arguments: expected at least 2, found 0
• cache: <HashMap> => error: expected ::, found )
• cache: std::collections::HashMap => wrong number of type arguments: expected at least 2, found 0

This is with Rust 1.0.0.beta.

Answer 1

Let's check out the compiler error message for this code:

use std::collections::HashMap;

fn fibbonacci(n: i32, cache: HashMap) -> i32 {}

fn main() {}

We get:

error[E0243]: wrong number of type arguments: expected at least 2, found 0
 --> src/main.rs:3:29
  |
3 | fn fibonacci(n: i32, cache: HashMap) -> i32 {}
  |                             ^^^^^^^ expected at least 2 type arguments

Note that it points directly to the issue and tells you that you need 2 type arguments. Rust requires that function arguments and return values be fully spelled out, there is no type inference at this point.

I don't know what you want the keys and values to be, so I'll assume i32:

fn fibonacci(n: i32, cache: HashMap<i32, i32>) -> i32 { 0 }

More verbosely, HashMap has two generic type parameters, referred to as K and V (but see note below). To reference a concrete type of HashMap, you need to specify what K and V are. You can also use more generic types but place trait bounds on the generics. This is a bit more advanced, and you don't need to worry about it to get started with Rust!

note - HashMap actually has 3 type parameters, but the third has a default value and isn't often used. That type parameter allows controlling the hashing algorithm used.