I can't respond directly to e12e's comment ("Please don't use equal for assignment" -- https://news.ycombinator.com/item?id=6704229) so I'll slightly abuse responding by doing so at this level.

We don't EVER use = for assignment. For us, = is binding. If you write

  fun f(x):
    y = x * x
    y = x + 2
    x - y
  end

Pyret will say

  I'm confused: y is defined twice

and point to the two bindings.

The goal here is to make the common case work fine, where you create a bunch of distinct local bindings; but when you try to mutate, you have to do it explicitly:

  fun f(x):
    var y = x * x
    y := x + 2
    x - y
  where:
    f(10) is -2 # the test passes
  end

The first line inside f says "y is _currently_ this, but it's variable, so look out!", and subsequent mutations use := to change y. Unlike JavaScript (say), mutating a variable that hasn't previously been defined is an error, rather than quietly adding the variable to the set of defined names.