How would I translate a Haskell type class into F#?

by

My brief answer is:

OO is not powerful enough to replace type classes.

The most straightforward translation is to pass a dictionary of operations, as in one typical typeclass implementation. That is if typeclass Foo defines three methods, then define a class/record type named Foo, and then change functions of

Foo a => yadda -> yadda -> yadda

to functions like

Foo -> yadda -> yadda -> yadda

and at each call site you know the concrete ‘instance’ to pass based on the type at the call-site.

Here’s a short example of what I mean:

// typeclass
type Showable<'a> = { show : 'a -> unit; showPretty : 'a -> unit } //'

// instances
let IntShowable = 
    { show = printfn "%d"; showPretty = (fun i -> printfn "pretty %d" i) }
let StringShowable = 
    { show = printfn "%s"; showPretty = (fun s -> printfn "<<%s>>" s) }

// function using typeclass constraint
// Showable a => [a] -> ()
let ShowAllPretty (s:Showable<'a>) l = //'
    l |> List.iter s.showPretty 

// callsites
ShowAllPretty IntShowable [1;2;3]
ShowAllPretty StringShowable ["foo";"bar"]

See also

https://web.archive.org/web/20081017141728/http://blog.matthewdoig.com/?p=112

Leave a Comment