Validation

Example Usage

First, we create some basic validation functions using the existing Result<_, _> type in F#.

Note that the Error case returns a string list. This is necessary.

let validateAge (age: int) =
    if age = 21
    then Ok 21
    else Error ["Invalid Age"]

let validateHeight (height: float) =
    if height = 180.
    then Ok 180.
    else Error ["Invalid Height"]

let validateName (name: string) =
    if name = "Alice"
    then Ok "Alice"
    else Error ["Invalid Name"]

Then, we create a composite data type that we want to validate.

type Data = {
    Age: int
    Height: float
    Name: string
    NotValidated: string
}

Now, we can use our validation computation expression to compose our existing validators.

let validate (data: Data) =
    validation {
        validate age in validateAge data.Age
        validate height in validateHeight data.Height
        validate name in validateName data.Name

        return {
            data with
                Age = age
                Height = height
                Name = name
        }
    }

We can now call our validate method:

validate {
    Age = 21
    Height = 180.
    Name = "Alice"
    NotValidated = "hello"
}

which will give us the following output:

Ok ({
    Age = 21
    Height = 180.
    Name = "Alice"
    NotValidated = "hello"
})

If we can validate with invalid data:

validate {
    Age = 20
    Height = 180.
    Name = "Bob"
    NotValidated = "hello"
}

we get the following result:

Error [
    "Invalid Age"
    "Invalid Name"
]

Take a look at sample.fsx or our unit tests for more examples.