Error handling

All expressions with the exception of collections can have an error value.

For instance:

let x: string = String.Read(Http.Get("http://broken/location"))

Because the URL http://broken/location is invalid, the String.Read returns an error.

To check if the string contains a value or an error, use:

let
    x: string = String.Read(Http.Get("http://broken/location"))
in
    Try.IsError(x) // true

It's also possible to build an error value. For instance, here is a function that returns the double of a number if it is less than 10, or an error otherwise.

let f(v: int) = if (v < 10) then v * 2 else Error.Build("v must be less than 10")

This function can be then be used as:

let
    f(v: int) = if (v < 10) then v * 2 else Error.Build("v must be less than 10")
in
    Try.IsSuccess(f(5)) // true

... while ...

let
    f(v: int) = if (v < 10) then v * 2 else Error.Build("v must be less than 10")
in
    Try.IsSuccess(f(100)) // false