CODEWITHSUNDEEP
f#quotations
Jon49
Jon49

Reputation: 4606

Get Field Value from Quotation and Cast to Its Type

I would like to get the typed value for a field from a record in a Quotation. It seems like it should be straight forward but I'm a bit lost.

E.g.,

type FullName = { First : string; Last : string }

type Name = { Name : FullName }

let t = { Name = { First = "Jon"; Last = "N" } }

let name = <@ t.Name.First @>

Then I would like to take the value name and get the Jon as a string (not an obj). How would I do this? Sometimes the return value might be an Array or another Record.

Thanks in advance!

Update:

I'll be using this function at the edges of F# so it needs to check for null:

let getValue (expr: Quotations.Expr<'t>) =
    match eval expr with
    | null -> None
    | x -> Some ((eval expr) :?> 't)

Upvotes: 2

Views: 178

Answers (1)

Fyodor Soikin
Fyodor Soikin

Reputation: 80744

The eval script that you linked will get you half way there - it will compute the actual value of the expression for you. Now all that's left to do is to cast that value to the appropriate type:

let getValue (expr: Quotations.Expr<'t>) = (eval expr) :?> 't

let valueOfName = getValue name  // valueOfName : string

Keep in mind that theoretically the cast may crash, but in practice that shouldn't happen, because eval would always return a value of the right type (unless there is a bug in it).

Upvotes: 4

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