How can I have a function that returns different types in F#?

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 bo开发者_StackOverflowok, 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.

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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.

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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.

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As an addition to previous answers, look at the Choice types (and Choice cases such as Choice1of3 etc.).