SML : how to use List.filter

list filter sml
hqt picture hqt · Feb 6, 2013 · Viewed 11.9k times · Source

I have this code to filter list of string that the first letter is capital:

fun f s = Char.isUpper(String.sub(s,0));
fun only_capitals (xs : string list) =  List.filter(f , xs);

But when compile, I always receive error :

operator domain: 'Z -> bool
operand:         (string -> bool) * string list
  in expression:
    List.filter (f,xs)

I don't know how to fix this. Can tell me, what does this error mean, and how to fix this.

Thanks :)

Answer

pad picture pad · Feb 6, 2013

Type signature of List.filter is

val filter : ('a -> bool) -> 'a list -> 'a list

So you need to give List.filter two distinct arguments, not one argument which happens to be a tuple.

Related questions

List comprehension vs. lambda + filter

I happened to find myself having a basic filtering need: I have a list and I have to filter it by an attribute of the items. My code looked like this: my_list = [x for x in my_list if …

Read More
remove None value from a list without removing the 0 value

This was my source I started with. My List L = [0, 23, 234, 89, None, 0, 35, 9] When I run this : L = filter(None, L) I get this results [23, 234, 89, 35, 9] But this is not what I need, what I really need is : [0, 23, 234, 89, 0, 35, 9] Because I'm calculating percentile of …

Read More
array filter in python?

For example, I have two lists A = [6, 7, 8, 9, 10, 11, 12] subset_of_A = [6, 9, 12]; # the subset of A the result should be [7, 8, 10, 11]; the remaining elements Is there a built-in function in python to do this?

Read More