The usefulness of lambdas becomes apparent when we combine two ideas:
- Lambdas are values just like Strings, Objects, and Booleans
- Values can be passed to functions as arguments, as well as returned from functions.
In other words, lambdas become useful when you want to pass functions as arguments to other functions, or return a function from a function.
Here’s an example of using a HOF, filter, to make sure an Array only contains odd numbers:
DW Script:
%dw 2.0
output json
fun isOddNum(n) =
(n mod 2) == 1
// Generate [1, 2, ..., 5]
var numbers = (1 to 5)
---
filter(numbers, (n, idx) -> isOddNum(n))
Output:
[1,3,5]
The filter function takes two arguments, an Array and a Lambda. In situations like these, it’s important to specify what the lambda should do as well. In the case of filter, the lambda should take in two arguments: an item in the Array, and the index of that particular item. It should return a Boolean. This Boolean value is used to determine if a value should be in the returned Array or not. It is the responsibility of the receiving function to pass arguments into the lambda you specified, and do something with the return value.
We had to give a name to the function (isOddNum) in order to use it, even though we were never going to use it again. This is exactly where lambdas come in. They enable us to pass functions to other functions without the inconvenience of having to think up a name for them:
DW Script:
%dw 2.0
output json
var numbers = (1 to 5)
---
filter(numbers, (n, idx) -> (n mod 2) == 1)
Output:
[1,3,5]
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