Lecture 4-2
Let
Sometimes we want to use the same expression more than once:
(x * x - 4) + sqrt (x * x - 4)
The problem of writing out the same equation over again can be solved using the following example:
let s = (x * x - 4) in s + sqrt s
Syntax
let <bindings> in <expression>
Where:
<binding>gives names to bindings separated by;let a = 1; b = a + 1 in a + b
<expression>uses those bindings
Let vs Variables
A let expression doesn’t create variables:
- They are names for particular expressions.
- You cannot change a binding once it has been made.
- They are made for convenience only.
Let in GHCI
In GHCI you can write:
> let a = 1
or
> a = 1
To define a let for the rest of the session.
Let across multiple lines
Usually it is clearer to write let across multiple lines:
f x y = let a = x * x
b = y * y
in
a * a + b * b
When splitting across line breaks you don’t need to use ; to separate the bindings.
Scope
When using let the bindings are only defined within the let function.
Examples
cylinder r h =
let sideArea = 2 * pi * r * h
topArea = pi * e ** 2
in sideArea + 2 * topArea
Exercises
-
exercise1 x = let a = x * x + 1 in a * a * a -
exercise2 x = let a = x + 1; bb = a + 2; ccc = a + bb in ccc * ccc
Haskell’s Layout Rule
Each definition at the same level should start on exactly the same column:
f x y z = let
a = x * x
b = y * y
cc = z * z
in
a * b + b * b
Ignoring the Layout Rule
You can ignore the layout rule by using curly braces to separate the bindings.
let {a = x * x;
b = y * y;
c = z * z}
in
a + b