Filter function in SML

问题

I use this function to filter a list of integers. I'm beggining in SML and I don't know where is the error.

fun filter f = fn [] => []
| fn (x::xs) => if f(x)
then x::(filter f xs) else (filter f xs)

fun g(x) = if x>5 then true else false

val listTest = filter g [1, 2, 4, 6, 8, 10] 

Thank you!

回答1:

Definitions with fn look like definitions with fun, but without the name and an arrow instead of =:

fn a0 => e0
 | a1 => e1
 | ...

This would be correct:

fun filter f = fn [] => []
                | (x::xs) => if f(x)
                             then x::(filter f xs) 
                             else (filter f xs)

but the common form is

fun filter _ [] = []
  | filter f (x::xs) = if f x
                       then x :: filter f xs
                       else filter f xs

fun g x = x > 5

回答2:

You second fn is redundant, please remove it:

fun filter f = fn [] => []
| (x::xs) => if f(x)
then x::(filter f xs) else (filter f xs)

Then it can compile happily:

- use "a.sml";
[opening a.sml]
val filter = fn : ('a -> bool) -> 'a list -> 'a list
val g = fn : int -> bool
val listTest = [6,8,10] : int list
val it = () : unit

BTW, fun g(x) = if x>5 then true else false is not good style. fun g x = x > 5 is better

来源：https://stackoverflow.com/questions/52824554/filter-function-in-sml