diff --git a/sml/week4/assigment.pdf b/sml/week4/assigment.pdf
new file mode 100644
index 0000000..154b7ba
Binary files /dev/null and b/sml/week4/assigment.pdf differ
diff --git a/sml/week4/hw3.sml b/sml/week4/hw3.sml
new file mode 100644
index 0000000..76e983f
--- /dev/null
+++ b/sml/week4/hw3.sml
@@ -0,0 +1,28 @@
+use "operators.sml";
+use "list.sml";
+
+fun first str = String.sub (str, 0)
+val only_capitals = filter (first >> Char.isUpper)
+
+val size = String.size
+fun choose f a b  = if f a b then a else b
+fun biggest a b = size a > size b
+fun bigger_or_equal a b = size a >= size b
+
+val longest_string1 = foldl (choose biggest) ""
+
+val longest_string2 = foldl (choose bigger_or_equal) ""
+
+fun longest_string_helper f =
+    let
+        val longest_string3 = longest_string1
+        val longest_string4 = longest_string2
+    in
+        if f (0, 0)
+        then longest_string4
+        else longest_string3
+    end
+
+val longest_capitalized = only_capitals >> longest_string2
+
+val rev_string = String.explode >> List.rev >> String.implode
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diff --git a/sml/week4/hw3_test.sml b/sml/week4/hw3_test.sml
new file mode 100644
index 0000000..3ac1460
--- /dev/null
+++ b/sml/week4/hw3_test.sml
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+use "test.sml";
+use "hw3.sml";
+
+
+val () =
+    assert
+    $ only_capitals ["Alpha", "beta"] = ["Alpha"]
+    $ "only_capitals: filter strings"
+
+val () =
+    assert
+    $ longest_string1 ["alpha", "betta"] = "alpha"
+    $ "longest_string1: finds longest string"
+
+
+val () =
+    assert
+    $ longest_string1 [] = ""
+    $ "longest_string1: empty syting on empty list"
+
+
+val () =
+    assert
+    $ longest_string2 ["alpha", "betta"] = "betta"
+    $ "longest_string2: on ties it returns the string closest to the end"
+
+val () = 
+    assert
+    $ longest_string_helper (op >) ["alpha", "betta"] = "alpha"
+    $ "longest_string_helper: a > b = longest_string1"
+
+val () = 
+    assert
+    $ longest_string_helper (op >=) ["alpha", "betta"] = "betta"
+    $ "longest_string_helper: a >= b = longest_string2"
+
+val () = 
+    assert
+    $ longest_capitalized ["Alpha", "bettaaaa"] = "Alpha"
+    $ "longest_capitalized: longest capitalized string"
+
+val () = 
+    assert
+    $ rev_string "string" = "gnirts"
+    $ "rev_string: reverses string"
+
+
+val () = complete ()
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diff --git a/sml/week4/list.sml b/sml/week4/list.sml
new file mode 100644
index 0000000..f5c4804
--- /dev/null
+++ b/sml/week4/list.sml
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+use "operators.sml";
+
+fun cons head tail = head :: tail
+
+fun foldl f acc lst =
+    case lst of
+    [] => acc
+    | head :: tail => foldl f (f head acc) tail
+
+fun reverse lst = foldl cons [] lst
+
+fun foldr f acc = foldl f acc >> reverse    
+
+fun map f = foldr (f >> cons) []
+
+fun filter predicate lst =
+    let
+        fun f elm acc = if predicate elm then elm :: acc else acc
+    in
+        foldr f [] lst
+    end
+
+fun empty lst = lst = []
+
+(* not efficient but works *)
+fun exists elem lst =
+     lst 
+     |> filter (fn needle => elem = needle) 
+     |> empty
+     |> not
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diff --git a/sml/week4/operators.sml b/sml/week4/operators.sml
new file mode 100644
index 0000000..1df007a
--- /dev/null
+++ b/sml/week4/operators.sml
@@ -0,0 +1,21 @@
+(* 
+    Pipe operator. 
+    Simply to write code like "data |> fun" instead "fun data".
+    Then you can "pipe" results to functions via partial application:
+    
+    fun square a b = a * b
+    fun sum a b = a + b
+
+    10 |> sum 10 |> square 2 // 40      
+*)
+fun op |> (x, f) = f x
+
+(* apply operator *)
+fun op $ (f, x) = f x
+
+(* composition operator *)
+fun op >> (f, g) x = g(f(x))
+
+infix |>
+infix $
+infix >>
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diff --git a/sml/week4/test.sml b/sml/week4/test.sml
new file mode 100644
index 0000000..8a4f8d4
--- /dev/null
+++ b/sml/week4/test.sml
@@ -0,0 +1,17 @@
+fun assert condition message = 
+    if condition 
+    then
+        print (message ^ "\n")
+    else 
+        let
+            val () = print ("Assert error: " ^ message ^ "\n")
+        in
+            OS.Process.exit OS.Process.failure
+        end
+
+fun complete () =
+    let
+        val () = print "All tests passed!"
+    in
+        OS.Process.exit OS.Process.success
+    end
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