F# generic types

Question

I am trying to use generics in an F# signature file, but I am having some troubles with the implementation.

this is the content of the signature file:

module BalancedBST 

type 'a BalancedBST 
 
val add : 'a -> 'a BalancedBST -> 'a BalancedBST  

and here is the implementation file:

module BalancedBST 

type SongName = SN of string 
type AlbumName = AlN of string 
type Year = Y of int16
type ArtistName = ArN of string 

type Song = SongName * AlbumName * Year * ArtistName 

type 'a BalancedBST = 
    | Leaf 
    | Node of Song * Song BalancedBST * Song BalancedBST
 
let  add (el : Song) (avl : Song BalancedBST) = avl  

As I am quite a newbie with this language and functional programming in general, I was expecting an OOP like behavior, where you define an interface with a generic type which will be specified later with the concrete class. Instead, I got this error while creating the dll:

Microsoft (R) F# Compiler version 10.2.3 for F# 4.5
Copyright (c) Microsoft Corporation. All Rights Reserved.

/path/to/fs/file error FS0034: Module 'BalancedBST' contains
    val add : SongName * AlbumName * Year * ArtistName -> avl:Song BalancedBST -> Song BalancedBST    
but its signature specifies
    val add<'a> : 'a -> 'a BalancedBST -> 'a BalancedBST    
The respective type parameter counts differ

Assuming that I undrestood that message properly, this means that the implementation file has to use functions that keep the generic inside their signature, which isn't something that I want.

Is there any solution to my problem or is it something that doesn't stick with functional programming and just belongs to OOP?

Answer 1

You can't have a generic signature file and a "more concrete" implementation that backs the signature this way. The add function must match the signature in the signature file.

If you want a generic interface with concrete implementations, you'll need to use an interface and back it with objects.