Haskell's "deriving Show" in F#?

Question

In Haskell it is easy to make an algebraic type/discriminated union "displayable" as a string by simply adding deriving Show to the type definition.

In F# I end up writing things like:

type Pos = 
    | Pos of int * int
    override this.ToString() = 
        match this with
        Pos(startp, endp) -> sprintf "Pos(%d, %d)" startp endp

and obviously it gets much worse with more complicated types.

Any way to get something like deriving Show in F#?

Err, F# already does this for you automatically so you don't have to write `deriving Show`. — J D, Dec 23 '10 at 17:52

score 20 · Accepted Answer · answered Feb 19 '10 at 11:37

F# printing functions such as printf are able to format reasonably any data type if you use the %A format specifier (they use ToString if you specify %O). You can implement ToString using sprintf which returns the formatted string:

type Pos =  
    | Pos of int * int 
    override x.ToString() = sprintf "%A" x

This prints for example "Pos (1, 2)" and it works for most of the F# types (lists, unions, records, tuples). It's a bit longer than just adding deriving Show but at least you don't have to implement the printing yourself.

and how could one achieve `deriving Read` in F#? – Cetin Sert Mar 16 '13 at 21:42 — Cetin Sert, Mar 16 '13 at 21:42

Haskell's "deriving Show" in F#?

1 Answers1