Can you run code before creating a type provider? (F#)

Question

Say:

let x = // some operation
type t = SomeTypeProvider<x>

Is this valid?

Answer 1

No.

Since the types must be generated at compile-time, the parameter to the type provider needs to be a constant.

In other words, the code you marked // some operation can evaluate to a literal, but cannot be a value returned by a runnable function:

let arg = "foo"
type t = SomeTypeProvider<arg> // okay

let [<Literal>] arg = """{"name":"foo","value":42}"""
type t = SomeTypeProvider<arg> // okay

let arg = x.ToString()
type t = SomeTypeProvider<arg> // Boom! arg is not a Literal

---

It depends on your application, but one of the most common cases is the following:

You have a database-related Type Provider, and the connection string needs to be retrieved in runtime, from some sort of config file or something. So a developer mistakenly thinks they need a runnable code to retrieve the connection string first and then pass it to the Type Provider.

The correct approach is the following:

Keep two databases: one locally stored in a constant location (just for schema), and another one for the runtime purposes.
Pass the first one (a constant!) to your Type Provider. Don't worry about the hardcoded paths; it is only used for schema retrieval.

// Use a fixed sample file for schema generation only
type MyCSVData = CsvProvider<"dummy.csv">

// Load the actual data at runtime
let data = MyCSVData.Load(RetrieveFileNameFromConfig())