idiomatic - Functional way to write these methods in F# -

in order calculate area of square , circle, defined following type:

type square = {width: float; length: float;}     member this.area = this.width * this.length     member this.perimeter = (this.width + this.length) * 2.  type circle = {r:float}      member this.area = system.math.pi * this.r * this.r     member this.perimeter = 2. * system.math.pi * this.r  let s1 = {width = 3.; length = 4.} let c1 = {r = 8.3}  printfn "%a" s1 printfn "the area of s1 is: %a" s1.area printfn "the perimeter of s1 is: %a" s1.perimeter   printfn "%a" c1 printfn "the area of c1 is: %a" c1.area printfn "the perimeter of c1 is: %a" c1.perimeter 

when read article: http://fsharpforfunandprofit.com/posts/type-extensions/

it states:

• methods don't play type inference
• methods don't play higher order functions

so, plea of new functionally programming. don't use methods @ if can, when learning. crutch stop getting full benefit functional programming.

then what's functional way solve problem? or what's idomatic f# way?

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edit:

after reading "the f# component design guidelines" (curtsy @v.b.), , @jacquesb's comment, consider implement member method within type simple, intrinsic way:

type square2 (width: float, length: float) =     member this.area = width * length     member this.perimeter = (width + length) * 2. 

(this identical original square type -- square2 saves seveal this. prefix in this.width, this.length.)

again, the f# component design guidelines quite useful.

a more functional way create shape discriminated union, square , circle cases. create functions area , perimeter, taking shape , using pattern matching:

type shape =     | square of width: float * length: float     | circle of r: float  let area = function     | square (width, length) -> width * length     | circle r -> system.math.pi * r * r  let perimeter = function     | square (width, length) -> (width + length) * 2.     | circle r -> 2. * system.math.pi * r  let s1 = square(width = 3., length = 4.) let c1 = circle(r = 8.3)  printfn "%a" s1 printfn "the area of s1 is: %a" (area s1) printfn "the perimeter of s1 is: %a" (perimeter s1)   printfn "%a" c1 printfn "the area of c1 is: %a" (area c1) printfn "the perimeter of c1 is: %a" (perimeter c1)