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# Local Definitions

We already saw how we can use

`where`

to define local helper expressions, but there is also another way. The `let`

-`in`

construct also allows us to define local expressions with the syntax `let <declarations> in <expression>`

:sumSquares2 x y =

let

a = x ^ 2

b = y ^ 2

in

a + b

ghci> sumSquares2 2 5

29

We can also use

`let`

-`in`

and `where`

in combination – for example, let's write a function that checks whether the sum of the squares of two numbers is a multiple of five:sumSquaresM5 x y =

let

sum = a + b

in

mod sum 5 == 0 -- (mod) is the modulo operator

where

a = x ^ 2

b = y ^ 2

We know a number is a multiple of five if the remainder of its division by five is zero. A thing to note is that there is a difference between

`let`

-`in`

and `where`

. What we define in `where`

declarations is accessible to any code above it. However, the declarations from the `let`

block are only accessible in the `in`

block of the same `let-in`

clause. For example, we could try to calculate the remainder from the division by five in the `where`

clause using the `sum`

from the `let`

clause, but it will not work:sumSquaresM5 x y =

let

sum = a + b

in

res == 0

where

a = x ^ 2

b = y ^ 2

res = mod sum 5 -- the "sum" expression from let-in is not accessible here

The compilation of the above would result in an error as

`sum`

is only accessible in the `in`

clause of the code. This means that with `let`

-`in`

we can create **super-localised expressions**that aren't accessible anywhere outside the`in`

code block.