- Extract Method
- Composed Method Pattern
- Single Responsibility Principal
Languages with higher-order functions
As a function gains complexity, its parts may get extracted out into named functions creating a deep stack of function calls. In these situations, it becomes the responsibility of the functions calling inner functions, to push data down the call stack.
This has 2 negative properties:
- Function have multiple responsibilities:
- To perform their intended calculation.
- To push data not intended for them down to the next function.
- When the call stack gets deep, it is harder to comprehend.
Remove the unwanted responsibility from the functions, then compose them to create a pipeline which transforms the data one step at a time.
(defn tax-multiplier [rate] (+ 1 (/ rate 100)))
(defn with-tax [rate amount] (* amount (tax-multiplier rate)))
; item-amounts has the single responsibility of extracting the amount properties
; from the items.
(defn item-amounts [items] (map :amount items))
; item-amounts-with-tax passes the items down to item-amounts and then adds
; the tax to exact of the amounts in the result.
(defn item-amounts-with-tax
[tax-rate item]
(map (partial with-tax tax-rate) (item-amounts items)))
(defn order-total
[tax-rate items]
(sum (item-amounts-with-tax tax-rate items)))(defn tax-multiplier [rate] (+ 1 (/ rate 100)))
(defn with-tax [rate amount] (* amount (tax-multiplier rate)))
; This function is fine
(defn item-amounts [items] (map :amount items))
; item-amounts-with-tax had two responsibilies. We've now replaced with this
; function which solely adds tax to a list of amounts
(defn amounts-with-tax
[tax-rate amounts]
(map (partial with-tax tax-rate) amounts))
; Now order-total simply composes all the steps to be applied to the items in
; in the order - one at a time
(defn order-total
[tax-rate items]
(->> items item-amounts amounts-with-tax sum))