Let<T> extension
let can be used to invoke one or more functions on results of call chains.
For example, the following code prints the results of two operations on a collection:
final numbers = ['one', 'two', 'three', 'four', 'five'];
final resultList = numbers.map((it) => it.length).where((it) => it > 3);
print(resultList);
With let that code can be rewriten:
final numbers = ['one', 'two', 'three', 'four', 'five'];
numbers.map((it) => it.length).where((it) => it > 3).let((it) {
print(it);
// And more function calls if needed.
});
let is often used for executing a code block only with non-null values.
To perform actions on a non-null object, use the
null aware operator
?. on it and call let with the actions in its block:
final String? str = 'Hello';
final length = str?.let((it) {
print('let() called on $it');
assert(it != null); // 'it' is not null inside '?.let()'
return it.length;
});
Another case for using let is introducing local variables with a limited scope for improving code readability.
let expects a function with a single argument, normally called it.
And this argument can be rename to anything:
final String? str = 'Hello';
final numbers = ['one', 'two', 'three', 'four'];
final modifiedFirstItem = numbers.first.let((firstItem) {
print('The first item of the list is "$firstItem"');
return iff(firstItem.length >= 5, () {
return firstItem;
}).orElse(() {
return '!$firstItem!';
});
}).toUpperCase();
print('First item after modifications: "$modifiedFirstItem"');
- on
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- T