Where and Let

Todo

Where clauses allow you to define sub-function (of sorts);

heron a b c = sqrt (s * (s - a) * (s - b) * (s - c))
    where -- important to note that it's indented
    s = (a + b + c) / 2

though this Wikibooks page gives a way better explanation.

The let binding allows for local declarations:

roots a b c =
    let sdisc = sqrt (b * b - 4 * a * c)
    in  ((-b + sdisc) / (2 * a),
         (-b - sdisc) / (2 * a))

let/in has a lot more to it than this, as one might expect.

Comparing where and let:

f = x+y where x=1; y=1 -- where is _not_ an expression and stays at the top level.

f = let x = 1; y = 2 in (x+y) -- let is an expression