Multiple ways you can do this. Most of these should also extend to multiple arguments, and although the constant is promoted to type level, you can pass it around nested functions as a type parameter.
With generics
In Java (personally I think this approach is best way to implement your specific example; also Kotlin, C#, and some others are similar):
interface HashAlgo<Options> {
String hash(String password, Options options);
}
class Bcrypt implements HashAlgo<BcryptOptions> { ... }
class Argon2 implements HashAlgo<Argon2Options> { ... }
record BcryptOptions { ... }
record Argon2Options { ... }
In Haskell without GADTs (also Rust is similar):
class HashAlgo opts where
hash :: String -> opts -> String
data BcryptOptions = BcryptOptions { ... }
data Argon2Options = Argon2Options { ... }
instance HashAlgo BcryptOptions where
hash password BcryptOptions { .. } = ...
instance HashAlgo Argon2Options where
hash password Argon2Options { .. } = ...
In C (with _Generic
):
typedef struct { ... } bcrypt_options;
typedef struct { ... } argon2_options;
char* hash_bcrypt(const char* password, bcrypt_options options) { ... }
char* hash_argon2(const char* password, argon2_options options) { ... }
#define hash(password, options) _Generic((options), bcrypt_options: hash_bcrypt, argon2_options: hash_argon2)(password, options)
In TypeScript, inverting which type is parameterized (see this StackOverflow question for another TypeScript approach):
type HashAlgo = 'bcrypt' | 'argon2'
type HashOptions<H> = H extends 'bcrypt' ? BcryptOptions : H extends 'argon2' ? ArgonOptions : never
function hash<H>(password: string, algorithm: H, options: HashOptions<H>): string { ... }
With constant generics or full dependent types
This way is a bit more straightforward but also way more complicated for the compiler, and most languages don't have these features or they're very experimental. Dependent types are useful when your constant is non-trivial to compute and you can't even compute it fully, like vectors with their length as a type parameter and append
guarantees the return vector's length is the sum. In that case generics aren't enough. Constant generics aren't full dependent types but let you do things like the vector-sum example.
In Haskell with GADTs AKA Generic Algebraic Data types (also works in Idris, Agda, and other Haskell-likes; you can simulate in Rust using GATs AKA Generic Associated Types, but it's much uglier):
data BcryptOptions = BcryptOptions { ... }
data Argon2Options = Argon2Options { ... }
data Algorithm opts where
Bcrypt :: Algorithm BcryptOptions
Argon2 :: Algorithm Argon2Options
hash :: String -> Algorithm opts -> opts -> String
hash password algo opts =
case algo of
| Bcrypt -> ... -- opts is inferred to be BcryptOptions here
| Argon2 -> ... -- opts is inferred to be Argon2Options here
In Coq (also flipping the parameterized types again):
Inductive algorithm : Set := bcrypt | argon2.
Inductive algorithm_options (A: algorithm) : Set := bcrypt_options : ... -> algorithm_options bcrypt | argon2_options : ... -> algorithm_options argon2.
Fixpoint hash (password : string) (algo : algorithm) (opts : algorithm_options also) : string := ... .