CODEWITHSUNDEEP
f#lexical-analysis
jeff
jeff

Reputation: 1078

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 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.

Upvotes: 3

Views: 1568

Answers (3)

Ramon Snir
Ramon Snir

Reputation: 7560

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

Upvotes: 1

Tomas Petricek
Tomas Petricek

Reputation: 243041

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.

Upvotes: 4

kvb
kvb

Reputation: 55184

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.

Upvotes: 13

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