# Testing for associativity

Recall that for scans and reductions, the
operator given must be *associative*. This means
that we can move around the parantheses in an application:

`(x `f` y) `f` z == x `f` (y `f` z)`

While it is undecidable for any compiler to determine statically
whether an arbitrary function `f`

is associative, itâ€™s not so hard
to write a test program that looks for a counterexample to
associativity by brute force.

```
def testassoc 'a [n] (eq: a -> a -> bool)
(op: a -> a -> a)
(ne: a)
(arr: [n]a) =let hom xs = reduce op ne xs
let golden = hom arr
in (.1) <| loop (ok, i) = (true, 0) while ok && i < n do
let (bef, aft) = (take i arr, drop i arr)
in if hom bef `op` hom aft `eq` golden
then (ok, i + 1) else (false, i)
```

It works by splitting the input `arr`

at every possible point,
reducing the two parts separately, combining these partial results
with `f`

, and then comparing them to reducing `arr`

as a whole. If
there is a difference (as determined with `eq`

), then we have a
counterexample.

For example, we can use it to show that subtraction is not associative:

```
> testassoc (==) (-) 0 [1,0,0,-1]
1i32
```

This says that reducing `[1]`

(the first 1 elements) and `[0,0,-1]`

(the remaining 3 elements) of the input array, then combining them
with `(-)`

, differs from reducing the whole test array in one go.
Ergo, something is wrong with the operator. If the operator is
indeed associative, then `testassoc`

returns the size of the array:

```
> testassoc (==) (+) 0 [0,0,0,2,2,2,2]
7i32
```

While capable of emitting false positives, in practice this technique works well, in particular when combined with large randomly generated input arrays.