# Pairwise L₁ distances

Source file: pairwise-l1.futFirst we define a function for computing the L₁ distance between two vectors:

```
let L1 [n] (xs: [n]f64) (ys: [n]f64) : f64 =
map2 (-) xs ys |> map f64.abs |> f64.sum
```

We use the forward pipe operator in the definition above.

To compute the pairwise L₁ distances between all rows in a matrix, we `map`

over the rows twice, then compute the distances between two rows:

```
let pairwise_L1 [n][m] (xss: [n][m]f64) : [n][n]f64 =
map (\a -> map (\b -> L1 a b) xss) xss
```

This pattern is very similar to matrix multiplication.