Operators and lists f#

Question

I have made this code:

let rec foo1 z = function
  | [] -> []
  | x::xs when x = z -> x::(foo1 x xs)
  | x::xs -> foo1 z xs

but I want a more complex version of this function with a new function which is of type int -> int -> bool as first argument. This function will test the integer argument against all the elements of the list, the elements that yields true when the operator is applied should be placed in the return list

Example:

let eq x y = x = y
let lt x y = x < y
let gt x y = x > y

foo2 eq 2 [1;2;4;2;5] => [2;2]
foo2 lt 2 [1;2;4;2;5] => [4;5]
foo2 gt 2 [1;2;4;2;5] => [1]

pad · Accepted Answer · 2012-11-27 13:23:27Z

4

/// Pass a function as an argument 
let rec filterByVal fn z = function 
    | [] -> [] 
    | x::xs when fn z x -> x::(filterByVal fn z xs) 
    | _::xs -> filterByVal fn z xs

/// Use anonymous functions
filterByVal (fun z x -> z = x) 2 [1;2;4;2;5] // [2; 2]
filterByVal (fun z x -> z < x) 2 [1;2;4;2;5] // [4; 5]
filterByVal (fun z x -> z > x) 2 [1;2;4;2;5] // [1]

/// Or use symbolic infix functions
filterByVal (=) 2 [1;2;4;2;5]
filterByVal (<) 2 [1;2;4;2;5]
filterByVal (>) 2 [1;2;4;2;5]

edited Nov 27, 2012 at 13:23

answered Nov 27, 2012 at 12:57

pad

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2 Comments

user1838768 Over a year ago

That I didn't know, do you know if there is any way to do the same thing but without defining the eq, lt and gt functions?

user1838768 Over a year ago

Can this only be done with integers or can it be anything else?

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