in smlnj how do you convert “string option” to “string”?

问题

Please help I have no idea how what a string option does.

is it possible to convert string option to a string?

回答1:

As already pointed out you could use pattern matching to get the desired result. So, something like this:

fun foo(NONE) = ""
  | foo(SOME a) = a;

But you could spare the trouble and use Option.valOf function from SML library instead by just doing:

Option.valOf(SOME "my string"); (Or just valOf(SOME "my string"); as newacct pointed out in the comments.)

回答2:

The "option" type in ML is like, say, Nullable in the .NET world. It is a discriminated union with two values, None and Some of 'a (for the generic 'a option type). To convert to a string, you need to get the value of 'a, which you can do via the normal pattern matching constructs.

Of course, if the value you have is None, there will be no string to retrieve. So, you need to handle the case where None appears, just as you might handle a null in other languages.

回答3:

for Example:

val x = SOME "String";

(*You can simply get it, by doing so: *)

val stringfromoptionstring = case x of SOME s => s | NONE => "No String found";

来源：https://stackoverflow.com/questions/6268700/in-smlnj-how-do-you-convert-string-option-to-string