To get the length of a list like

`length [1..10] == 10`

You have to walk the whole list to check that it’s indeed of length `10`

.

Another way is to define a natural number in peano arithmetic style:

```
data Nat = Zero | Add1 Nat
Ord, Eq) deriivng (
```

So

```
-- 0 = Zero
-- 1 = Add1 Zero
-- 2 = Add1 (Add1 Zero)
-- 3 = Add1 (Add1 (Add1 Zero))
```

We define a trivial instance of `Num`

for it:

```
instance Num Nat where
Zero + y = y
Add1 x + y = x + (Add1 y)
fromInteger 0 = Zero
fromInteger n = Add1 (fromInteger (n - 1))
```

Now we can use `genericLength`

on a list like this. Equal-sized lists:

```
> genericLength [1..3] == (genericLength [1..3] :: Nat)
True
```

With infinite lists:

```
> genericLength [1..3] < (genericLength [1..] :: Nat)
True
> genericLength [1..3] == (genericLength [1..] :: Nat)
False
> genericLength [1..3] > (genericLength [1..] :: Nat)
False
```

We didn’t have to walk the whole list to compare its length with a finite number. That’s pretty cool.