This value is not a function and cannot be applied

Question

Pretty simple function for operation of negation.

let negation (value:option<bool>) =
   match value with
   |Some true -> Some false
   |Some false -> Some true
   |None -> failwith "OOPS"

but when i call it:

negation Some true 

it complains that

This value is not a function and cannot be applied

Answer 1

You need some parens there:

negation (Some true)

Or:

negation <| Some true

Without the parens like that F# compiler would understand that line as

(negation Some) true

because function application is left-binding and then types are not match: negation would need to be of type: ('a -> option 'a) -> bool -> bool which clearly isn't (is of type bool option -> bool option)

In addition: (opinion included)

Negation function is called not : bool -> bool. You are trying to use that on the bool wrapped by option, so maybe that should be sufficient:

let negation : bool option -> bool option = Option.map not