list intersection

intersection.fsx

//a: how would you compute intersection of two lists?
//b: I'd use set intersection, but if literally a list, I'd sort then merge:

let intersection a b =
    let rec intersection' a b =
        match a,b with
        | _,[] -> []
        | [],_ -> []     
        | x::a',y::b' when x=y -> x::intersection' a' b' 
        | x::a',y::_ when x<y -> intersection' a' b
        | x::_,y::b' when x>y -> intersection' a b'
    intersection' (List.sort a) (List.sort b)

    
[
  intersection [] [] = []
  intersection ['a'] ['a'] = ['a']
  intersection ['a'] ['b'] = []
  intersection ['a';'b'] ['a';'b'] = ['a';'b']
  intersection ['a';'c'] ['c'] = ['c']
  intersection ['c'] ['a';'c'] = ['c']
  intersection ['b'] [] = []
  intersection [] ['b'] = [] 
  intersection ['a';'b';'c'] ['a'] = ['a']
  intersection ['a'] ['a';'b';'c'] = ['a']
  
  intersection ['a';'b'] ['b';'a'] = ['a';'b']
  
  intersection ['a'..'d'] ['b'..'g'] = ['b'..'d']

]