Menu
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#?
787
The second line, deriving (Eq, Show) , is called the deriving clause; it specifies that we want the compiler to automatically generate instances of the Eq and Show classes for our Pair type. The Haskell Report defines a handful of classes for which instances can be automatically generated.
The shows functions return a function that prepends the output String to an existing String . This allows constant-time concatenation of results using function composition.
An instance of a class is an individual object which belongs to that class. In Haskell, the class system is (roughly speaking) a way to group similar types. (This is the reason we call them "type classes"). An instance of a class is an individual type which belongs to that class.
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.
183
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
