Toplevel expression is expected to have unit type

[jcsherin]

You encounter this error (in chapter 2) before the unit type is introduced. Introduce the underscore binding to which you can assign values, you do not care about. This will solve the compiler error.

Also better explain why this is a compilation error in the first place. Explain the term toplevel expression.

[stevez]

I found a quick solution is assigning the result to a variable, then the error will be disappeared, like this
let x = Js.Array.map(tupleToString, languagesAndAuthors)

[seho-dev]

let addCoordinates = (~x, ~y) => x + y
let a = addCoordinates(~x=1, ~y=2)