SML Pattern matching throwing "types of rules don't agree [tycon mismatch]" error -

SML Pattern matching throwing "types of rules don't agree [tycon mismatch]" error -

fun sample(list_of_str_lists, s) = case list_of_str_lists of [] => [] | x::[] => case (all_except_option(s, x)) of lst => lst | none => [] | x::xs' => case (all_except_option(s, x)) of lst => lst @ sample(xs', s) | none => [] @ sample(xs', s)

it uses helper function takes list of strings , returns elements in list in list option if there matching string in list (again, excluding matching string). helper function take list ["a", "b", "c"] , string "a" , if matches "a", homecoming alternative containing ["b", “c”].

my problem in sample in lastly pattern matching branch. throws error

earlier rule(s): 'z list alternative -> 'z list rule: string list list -> string list in rule: :: (x,xs') => (case (all_except_option (s,x)) of lst => lst @ sample <exp> | none => nil @ sample <exp>

but don’t understand why, because pattern i’m trying match calls some lst it’s describing string list list. what’s going on?

this syntax issue due nested case expressions. parser assumes case | x::xs' => ... continuation of ongoing case look case (all_except_option(s, x)) of ... since has no way of telling it's meant belong enclosing look case list_of_str_lists of ....

if enclose inner case expressions in parentheses, function definition parse.

fun sample(list_of_str_lists, s) = case list_of_str_lists of [] => [] | x::[] => (case (all_except_option(s, x)) of lst => lst | none => []) | x::xs' => (case (all_except_option(s, x)) of lst => lst @ sample(xs', s) | none => [] @ sample(xs', s))

pattern-matching sml