Question

Trying to figure out how to create a method that takes in a list of methods and another parameter. I have my method that I created in C# that I’m trying to carry over to F#:

public interface IValidator
{
    MethodResult Validate(IList<Func<userModel, MethodResult>> validationMethods
      , userModeltoValidate);
}

I’ve tried something like:

type public IValidator =
  abstract Validate : IList<(BasicUserModel -> MethodResult<'a>)>, BasicUserModel 
    -> MethodResult<'a>

But it doesn’t like the syntax even if I surround the parameter signature with parentheses. The end goal is to have a method that takes in a list of methods and run them by iterating through the list.

public MethodResult Validate(IList<Func<BasicUserModel, MethodResult>> validationMethods, 
   BasicUserModel toValidate)
{
   ...
}

The looping part I can get easily with F#, it’s the method signature syntax for both the abstract type and the type that’s implementing the method that’s killing me.

Answer 1

In the abstract method declaration, rather than separating the argument types with commas they should be separated by asterisks, as if the method took a tuple:

abstract Validate : IList<(BasicUserModel -> MethodResult<'a>)> * BasicUserModel 
                         -> MethodResult<'a>  

Here are a few other thoughts:

1. Types are public by default, so you can omit the public keyword on your type declaration.
2. It would be more idiomatic (and more flexible for callers) to take a seq<_> than an IList<_>, unless you’re likely to need random access rather than looping.
3. I assume it’s intentional, but your F# validate method will be generic (since it returns a MethodResult<'a> using an undeclared type parameter 'a). If you want a non-generic method on a generic type, then add the 'a parameter to the type declaration: type IValidator<'a> = ....