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union arrays and zero_create
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Maybe I'm going about this the wrong way, but I'm having some trouble properly initializing an array of types defined as a union. If I have a union like this:

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type Thing =
| ThingInt of int
| ThingFloat of float

And then I make an array with zero_create, like so:

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let t = (Array.zero_create 10 : Thing[])

The array comes back populated with nulls. That makes a reasonable amount of sense, the union doesn't have any kind of default value or constructor. Is it possible to specify what value a union initializes as? Failing that, is there a way to explicitly create the array with a particular value? This doesn't seem to work, even though I feel like it should:

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let t = (Array.create 10 : Thing[])
let t = (Array.init 10 : Thing[])

What have I missed?

DC

.
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let t = Array.create 10 (ThingInt 0)


let t = Array.init 10 (fun i -> ThingInt i)

When possible, avoid Array.zero_create. Getting null values is always dangerous.

Laurent.

By on 7/24/2007 11:54 AM ()

Perfect. A collection full of null values scares the red out of my beard.

DC

By on 7/24/2007 1:11 PM ()

Actually, after playing around, that only allows me to make an array of one particular thing out of the union. If I have the union previously defined, I can't seem to make an array that will hold either kind of Thing. Is that possible?

DC

By on 7/24/2007 1:19 PM ()

OK, here are two examples:

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let t = Array.create 5 (ThingInt 0);;

val t : Thing array

> t.[0] <- ThingFloat 4.2;;

val it : unit = ()

Since t is a Thing array, you can store either ThingInt or ThingFloat.

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> let init_fct x =

  if x % 2 = 0 then ThingFloat 1.1

  else ThingInt x;;

val init_fct : int -> Thing

> let t = Array.init 5 init_fct;;

val t : Thing array

The initialization function can return any kind of Thing.

Keep in mind the type of t is Thing (and not ThingInt or ThingFloat: they are only type constructors). That means t can be any value of type Thing.

Laurent.

By on 7/24/2007 5:30 PM ()

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