sml - Type Mismatch in ML List.filter -

i trying write simple filter function in ml. idea function only_capitals takes list of strings , returns list of strings, strings start capital letter. here implementation, getting type-error not understand:

fun only_capitals (strs : string list) =    let      fun iscapitalized (str) = char.isupper(string.sub(str, 0))    in      list.filter(iscapital, strs)    end 

here error:

hw3provided.sml:5.18-5.27 error: unbound variable or constructor: iscapital hw3provided.sml:5.6-5.34 error: operator , operand don't agree [tycon mismatch]   operator domain: 'z -> bool   operand:         _ * string list   in expression:     list.filter (<errorvar>,strs) val = () : unit 

the first error caused typo; "iscapital" not name of function defined.

the second error looks strange because of first error – type _ refers type of iscapital.
if fix first error, second should more like

error: operator , operand don't agree [tycon mismatch]   operator domain: 'z -> bool   operand:         (string -> bool) * string list   in expression:     list.filter (iscapitalized,strs) 

what compiler trying you're passing pair (iscapitalized,strs) filter expects function of type 'z -> bool.

if look @ type of list.filter, you'll notice ('a -> bool) -> 'a list -> 'a list – it's curried function.

what should write is

fun only_capitals (strs : string list) =    let      fun iscapitalized (str) = char.isupper(string.sub(str, 0))    in      list.filter iscapitalized strs    end