CODEWITHSUNDEEP
typesf#pattern-matchingdiscriminated-union
user10632986
user10632986

Reputation:

Returning different state of Discriminate union based upon string input in pattern match (F#)

type ColType = I of int | S of string | F of float

I know that you cannot return a different type from a pattern match which is why I made this type in the first place for a parser I am creating. However with this code:

 //create i coltype
    let m (x : int) : ColType =
        ColType.I x
    //create f coltype
    let n (x : float) : ColType = 
        ColType.F x
    //create s coltype
    let b (x : string) : ColType = 
        ColType.S x    

    let pColType =

        let innerFn (charList : char list) = 

            let i = String(List.toArray charList)
            match i with   
            | "int"  -> m
            | "float" ->  n
            | "string" -> b

        many1 parseLowercase |>> innerFn

It will still give me an error in the pColType function despite m, n and b returning the same type.

code such as parseLowercase etc is just code to get a string, this is functioning as intended, the problem is the return value not being the same despite all being coltype? (altough different states of it).

Upvotes: 1

Views: 42

Answers (1)

AMieres
AMieres

Reputation: 5004

Like rmunn and Romanov mentioned your 3 functions are of different type. You can make them return a single type for instance obj -> ColType using box & unbox:

let pColType =

    let innerFn (charList : char list) = 
        let i = String(List.toArray charList)
        match i with   
        | "int"    -> unbox >> m
        | "float"  -> unbox >> n
        | "string" -> unbox >> b
        |_-> failwithf "error %A" i

    many1 parseLowercase |>> innerFn

That requires that you box the value before invoking the resulting function. Since you are building a parser it makes more sense to instead make it return a string -> ColType:

let pColType =

    let innerFn (charList : char list) = 
        let i = String(List.toArray charList)
        match i with   
        | "int"    -> parseInt    >> m
        | "float"  -> parseFloat  >> n
        | "string" -> parseString >> b
        |_-> failwithf "error %A" i

    many1 parseLowercase |>> innerFn

Upvotes: 1

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