Subtract

Subtract is a curried constructor used to create a JS/JSON configuration object that represents a subtraction operation.

const Subtract =
  (datatype = "Number") =>
  minuend =>
  subtrahend => ({
    tag: "Subtract",
    minuend,
    subtrahend,
    datatype,
  })
 
export default Subtract
 

The first parameter to Subtract determines the datatype of the return value of the subtraction operation: Number (float) or Integer.

The second parameter is the minuend; the third is the subtrahend. Both are operations, which must eventually evaluate to values. The subtrahend will eventually be subtracted from the minuend to produce the difference (result).

The minuend and subtrahend operations (multiply, subtract, etc.) may be nested to any depth, but the bottom (leaf) nodes must return values. Values are injected with injectors.

Because Subtract is curried, we can simplify our code by partial application, for example, const Quotient = Subtract("Integer"). This returns a function that takes the minuend and subtrahend operations and returns a Subtract configuration for working with integers.

See below for an example.

The subtract function

We pass our configuration object to composeOperators, which composes the operations and returns a single function. This function takes an optional argument and returns the result of the calculation as a Right, e.g., { right: 42 }, or a Left with an array of Error objects, { left: [Error] }.

const subtract = operation => (arg? => Either<Array<Error>, result>)

composeOperators works by recursing down through the operation object, calling the correct operator function based on the tag (e.g., "Subtract"), and composing the functions returned. composeOperators then returns this composed function. See composeOperators for a more detailed explanation.

The injectors are not called until this composed calculate function is run. Hence, evaluation is lazy: the values are not injected until the last moment.

See the injectors for a complete list of how values may be injected. See the list of operators for the full range of available mathematical operations.

Example

We use composeOperators to create a calculate function, passing it our Subtract operation object. We call the calculate function and pass it an (optional) argument to run our operation(s) and calculate the difference.

Here our minuend operation is a nested division operation of two constants (using Constant). Our subtrahend is also an operation that returns the argument passed to the composed function (using FromArgument).

Try leaving one of the numbers undefined to see an Error returned.