Lecture 9-1

COMP105 Lectures 2020-10-29

where Syntax

Sometimes it is useful to bind names across a whole function:

remove_twos [] = []
remove_twos (x:xs)
	| x == 2	= rest
	| otherwise = x:rest
	where rest = remove_twos xs

As the two expressions are in different guards then you can’t use a let expression.

A general example is:

f x y = 1 + a + b
	where 	a = 2 * x
			b = y / 2

where comes at the end of a function. Like let it can bind any number of names

• These names are available throughout the function.
• They are not variables.
• You can do pattern matching in where clauses (and also in let expressions too).

An example of pattern matching:

initials first last = (x,y)
	where	(x:_) = first
			(y:_) = last

let v.s. where

let is an expression.

> (let a = 1 in a) + (let a = 2 in a)
> 3

where is syntax

• One where clause per function.
• Particularly useful when used with guards.

remove_twos [] = []
remove_twos (x:xs)
	| x == 2	= rest
	| otherwise = x:rest
	where rest = remove_twos xs	

This means that where has to be joined to a function body and is always bound to that function body.

Exercises

1.  func x = a + b + x
        where 	a = 2 * x + 1
                b = a * a - x
   

2.  countTwos [] 	= 0
    countTwos (x:xs)
        | x == 2	= 1 + rest
        | otherwise = rest
        where rest = countTwos xs