如何在F＃中使用返回不同类型的函数？

Question

I've made a scanner in F#. Currently it returns a list of bunch of tuples with type (Token, string).

Ideally I'd like to return a list of tuples that might contain different types. For example:

(Token, string) 
//if it's an identifier

(Token, float)
//if it's a float. 

(Token, int)
//etc

So, Basically I'd like to return type (Token, _) but I'm not sure how to specify this. Right now it just has errors complaining of mismatched types. I'm looking through my book, and wikibooks, but I'm not really sure what this is called.

If this really isn't possible, I guess I can convert the types later on, but I was hoping I could just return stuff this way.

Answer 1

There are two relatively easy ways to handle this in F#. One would be to create a discriminated union:

type Expression =
| Identifier of string
| FloatValue of float
| IntValue of int
| ...

and then define your function so that it returns a (Token * Expression) list . The other possibility is to box everything into an object and then return a (Token * obj) list . This is slightly easier for the producer of the list, but much more annoying for the consumer.

Answer 2

I think that using discriminated union as kvb suggests is the way to go. Just to add some more information, when writing scanners in F#, it is common to define a token type Token that lists various types of tokens that may carry additional information like this:

type Token = 
// Some toknes with no additional information
| LParen | RParen | Begin | End
// Some tokens with additional values
| Identifier of string
| IntValue of int
| ...

Then you completely remove the need for representing the value separately from the token and you can work with just Token list .

Answer 3

作为以前答案的补充，请查看Choice类型（以及Choice1of3等选择案例）。