# Factorial

### via User-defined Postfix Operator

`[*]` is a reduction operator that multiplies all the following values together. Note that we don't need to start at 1, since the degenerate case of `[*]()` correctly returns 1, and multiplying by 1 to start off with is silly in any case.

```perl
sub postfix:<!> (Int $n) { [*] 2..$n }
say 5!;
```

#### Output:

```
120
```

### via Memoized Constant Sequence

This approach is much more efficient for repeated use, since it automatically caches. `[\*]` is the so-called triangular version of \[\*]. It returns the intermediate results as a list. Note that Raku allows you to define constants lazily, which is rather helpful when your constant is of infinite size...

```perl
constant fact = 1, |[\*] 1..*;
say fact[5]
```

#### Output:

```
120
```