Haskell: let..in block

Most of the times we need intermediate results to get the final outcome, By using let..in block we can define intermediate results in let block and perform main calculations in ‘in’ block.

Suppose if a triangle has sides a, b and c. As per Herons’s formual, following formula is used to calculate area of triangle.

Where s is semi perimeter.

triangle.hs

areaOfTriangle a b c = 
    let s = (a + b + c) / 2
    in sqrt (s * (s - a) * (s - b) * (s - c))

Prelude> :load triangle_area.hs
[1 of 1] Compiling Main             ( triangle_area.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> areaOfTriangle 40 40 20
387.2983346207417

Following application find the roots of polynomial application.

Following is the solution for polynomial application.

roots.hs

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

*Main> :load roots.hs
[1 of 1] Compiling Main             ( roots.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> roots 10 100 5
(-5.0252531694167143e-2,-9.949747468305834)

You can declare multiple variables in let clause.

letEx.hs

processData x = let a = 10
                    b = 20
                in if (x < 0) then (a+b)
                   else (a+b*x)

Prelude> :load letEx.hs
[1 of 1] Compiling Main             ( letEx.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> processData 0
10
*Main> 
*Main> processData 1
30
*Main> processData 2
50
*Main>

Shadowing

We can insert a let block inside another let block, If we repeat the same variable name in inner let expression, it hides the variable defined in outer let expression.

letEx.hs

processData x = let a = 10
                    b = 20
                in if (x < 0) then let a = 20 in (a, b)
                   else let b = 30 in (a, b)

*Main> :load letEx.hs
[1 of 1] Compiling Main             ( letEx.hs, interpreted )
Ok, modules loaded: Main.
*Main> 
*Main> processData (-10)
(20,20)
*Main> 
*Main> processData 10
(10,30)
*Main> 

When I call the function processData (-10), it returns (20, 20), since variable a is assigned to 20 in inner let block, which hides a=10 in outer let block.

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