Create README.md

README.md

@@ -0,0 +1,14 @@
+# Programming Languages
+my solutions to ["Programming Languages"](https://www.coursera.org/learn/programming-languages) course by Dan Grossman. 
+
+Course consists of three separate parts in which author tries to explain language concepts / paradigms through different languages:
+- SML
+- Raket
+- Ruby
+
+Each part has exercises for specific language. You can find this exercies and my solutions in repo.
+
+### TODO
+- ~~SML~~
+- Raket
+- Ruby

sml/week2/date.sml

@@ -1,4 +1,24 @@
-use "operators.sml";
+(* 
+    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 |> (x, f) = f x
+
+(* apply operator *)
+fun $ (f, x) = f x
+
+(* composition operator *)
+fun >> (f, g) x = g(f(x))
+
+infix |>
+infix $
+infix >>
 
 (* year * month * day *)
 type Date = int * int * int
@@ -71,6 +91,7 @@ fun uniqe lst =
     end
 
 (* naive sort, will blow up the stack. Can't use pattern matching for now :( *)
+(* also incorrect *)
 (* fun sort f lst =
     case lst of
     [] => []
@@ -81,7 +102,7 @@ fun uniqe lst =
         else first :: second :: sort f rest *)
 
 (* also will blow up the stack *)
-fun sort f lst =
+fun sort' f lst =
     if lst = []
         then []
         else
@@ -95,18 +116,25 @@ fun sort f lst =
                     val greater = f first second
                 in
                     if greater
-                    then second :: first :: sort f rest
-                    else first :: second :: sort f rest
+                    then second :: sort' f (first :: rest)
+                    else first :: sort' f (second :: rest)
                 end
 
+(* simple recursive convergence *)
+fun fix f g x = 
+    if f g x = x 
+    then x 
+    else fix f g (f g x)
+
+(* naive bubble sort *)
+fun sort f x = fix sort' f x
+
 fun is_older ((y1, m1, d1): Date, (y2, m2, d2): Date): bool =
         let
-            val same_dates = y1 = y2 andalso m1 = m2 andalso d1 = d2
-            val older = y1 <= y2 andalso m1 <= m2 andalso d1 <= d2
+            val first_days = y1 * 360 + m1 * 30 + d1
+            val second_days = y2 * 360 + m2 * 30 + d2
         in
-            if same_dates
-            then false
-            else older
+            first_days < second_days
         end
 
 fun number_in_month (dates: Date list, month_to_find: int): int =
@@ -170,16 +198,12 @@ fun date_to_string ((year, month, day): Date): string =
             "December"
         ]
 
-        val string_month = get_nth (months, month - 1)
+        val string_month = get_nth (months, month)
 
         val to_str = Int.toString
 
-        fun pad num =
-            if num < 10
-            then "0" ^ to_str num
-            else to_str num
     in
-        string_month ^ " " ^ pad day ^ ", " ^ to_str year
+        string_month ^ " " ^ to_str day ^ ", " ^ to_str year
     end
 
 fun number_before_reaching_sum (sum: int, numbers: int list): int =

sml/week2/date_tests.sml

@@ -17,6 +17,11 @@ val () =
         (sort (fn a => fn b => a > b)  [2, 1] = [1, 2]) 
         "sort: two elements"
 
+val () =
+    assert
+        (is_older ((1,2,25), (6,7,8)) = true)
+        "is_older: first date older than second by year and month" 
+
 val () =
     assert
         (is_older ((1999, 12, 31), (1999, 12, 31)) = false)
@@ -185,7 +190,12 @@ val () =
 
 val () =
     assert
-        (date_to_string ((2020, 06, 01)) = "May 01, 2020")
+        (date_to_string (2020, 06, 01) = "June 1, 2020")
+        "date_to_string: returns correct string"
+
+val () =
+    assert
+        (date_to_string (3, 1, 1) = "January 1, 3")
         "date_to_string: returns correct string"
 
 val () =
@@ -258,6 +268,14 @@ val () =
         assert (oldest dates = expect) "oldest: returns oldest"
     end
 
+val () =
+    let 
+        val dates = [(5,5,2), (5, 10, 2), (5, 2, 2), (5, 12, 2)]
+        val expect = SOME (5, 2, 2)
+    in
+        assert (oldest dates = expect) "oldest: returns oldest"
+    end
+
 val () =
     let
         val dates = [(2000, 11, 31), (2000, 11, 31)]

sml/week3/card.sml

@@ -31,7 +31,7 @@ val remove_card: remove_card = fn (cards, to_remove, exp) =>
             then (found, card :: acc)
             else (true orelse found, acc)
     in
-        case fold filter (false, []) cards of
+        case foldl filter (false, []) cards of
         (true, filtered) => filtered
         | (false, _) => raise exp
     end
@@ -47,10 +47,11 @@ val rec all_same_color: all_same_color = fn cards =>
 
 type sum_cards = card list -> int
 val sum_cards: sum_cards = fn cards =>
-    cards |> fold (fn card => fn sum => card_value card + sum) 0 
+    cards |> foldl (fn card => fn sum => card_value card + sum) 0 
 
+fun sum a b = a + b
 (* even shorter via partial application :) *)
-val sum_cards: sum_cards = fold (fn card => fn sum => card_value card + sum) 0 
+val sum_cards: sum_cards = foldl (card_value >> sum) 0 
 
 type score = card list * int -> int
 val score: score = fn (cards, goal) =>
@@ -83,5 +84,5 @@ val officiate: officiate = fn (deck, moves, goal) =>
         
         fun round hand = score (hand, goal)
     in
-        moves |> fold play (deck, []) |> #2 |> round 
+        moves |> foldl play (deck, []) |> #2 |> round 
     end

sml/week3/card_tests.sml

@@ -212,4 +212,11 @@ val 0 =
                 |> (fn _ => expected)
     end
 
+fun by2 a = a * 2 
+
+val () =
+    assert
+    $ map by2 [1, 2, 3] = [2, 4, 6]
+    $ "map test"
+
 val () = complete ()

sml/week3/list.sml

@@ -2,23 +2,22 @@ use "operators.sml";
 
 fun cons head tail = head :: tail
 
-fun fold f acc lst =
+fun foldl f acc lst =
     case lst of
     [] => acc
-    | head :: tail => fold f (f head acc) tail
+    | head :: tail => foldl f (f head acc) tail
 
-fun reverse lst =
-    let
-        fun f elm acc = elm :: acc
-    in
-        fold f [] lst
-    end
+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
-        lst |> fold f [] |> reverse
+        foldr f [] lst
     end
 
 fun empty lst = lst = []
@@ -27,5 +26,5 @@ fun empty lst = lst = []
 fun exists elem lst =
      lst 
      |> filter (fn needle => elem = needle) 
-     |> empty 
+     |> empty
      |> not

sml/week3_2020/json.sml

@@ -0,0 +1,106 @@
+use "operators.sml";
+use "list.sml";
+
+exception NotSorted
+
+datatype json =
+    Num of real
+    | String of string
+    | False
+    | True
+    | Null
+    | Array of json list
+    | Object of (string * json) list
+
+val strcmp = String.compare
+
+type make_silly_json = int -> json
+val make_silly_json: make_silly_json = fn num =>
+    let
+        fun construct num json =
+            case num of
+            0 => json
+            | _ => 
+                case json of 
+                Array arr => Array $ arr @ [Object [("b", True), ("n", Num $ Real.fromInt num)]]
+    in
+        construct num (Array [])
+    end
+
+(* not tail recursive *)
+type concat_with = string * string list -> string
+val rec concat_with: concat_with = fn (separator, strings) =>
+    case strings of
+        [] => ""
+        | one :: [] => one
+        | first :: second :: rest => 
+            first ^ separator ^ second ^ concat_with (separator, rest)  
+
+(* complex, two iterations but tail recursive :) *)
+val rec concat_with: concat_with = fn (separator, strings) =>
+    let
+        fun insert elem acc = elem :: separator :: acc
+        fun concat elem acc = elem ^ acc 
+    in
+        case strings of
+        [] => ""
+        | one :: [] => one
+        | head :: tail => foldl insert [head] tail |> foldl concat ""
+    end  
+
+type quote_string = string -> string
+val quote_string: quote_string = fn str => "\"" ^ str ^ "\""
+
+type real_to_string_for_json = real -> string
+val real_to_string_for_json: real_to_string_for_json = fn num =>
+    if num < 0.0 
+    then "-" ^ (Real.toString (~num)) 
+    else Real.toString num
+
+type json_to_string = json -> string
+val rec json_to_string: json_to_string = fn json =>
+    let
+        fun concat strs = concat_with (",", strs)
+
+        fun convert f json = json |> foldr f [] |> concat
+
+        fun array elem all = json_to_string elem :: all
+
+        fun object (key, value) all = 
+            (quote_string key ^ ":" ^ json_to_string value) :: all
+    in
+        case json of
+        Num num => real_to_string_for_json num
+        | String str => quote_string str
+        | False => "false"
+        | True => "true"
+        | Null => "null"
+        | Array json => "[" ^ convert array json  ^ "]"
+        | Object json => "{" ^ convert object json ^ "}"
+    end 
+
+type (''a, 'b) assoc = ''a * (''a * 'b) list  -> 'b option
+val rec assoc = fn (needle, lst) =>
+    case lst of
+    [] => NONE
+    | (key, value) :: tail =>
+        if key = needle 
+        then SOME value 
+        else assoc (needle, tail)
+
+type count_occurrences = (string list * exn) -> (string * int) list
+val rec count_occurrences: count_occurrences = fn (strs, exn) => 
+    let
+        fun repetitons str lst reps order acc =
+            case lst of
+            [] => acc @ [(str, reps)] 
+            | head :: tail => case strcmp (str, head) of
+                EQUAL => repetitons str tail (reps + 1) order acc
+                | new_order =>  if order <> EQUAL andalso new_order <> order
+                                then raise exn
+                                else repetitons head tail 1 new_order (acc @ [(str, reps)])
+    in
+        case strs of 
+        [] => [] 
+        | head :: tail => repetitons head tail 1 EQUAL []
+    end

sml/week3_2020/json_test.sml

@@ -0,0 +1,122 @@
+use "test.sml";
+use "operators.sml";
+use "json.sml";
+
+val () =
+    case make_silly_json 1 of
+       Array [Object [("b", True), ("n", (Num _))]] => 
+        assert true "make_silly_json: pass"
+     | _ => 
+        assert false "make_silly_json"
+
+val () =
+    assert
+    $ concat_with (":", []) = ""
+    $ "concat_with: [] = ''"
+
+val () =
+    assert
+    $ concat_with (":", ["a", "b"]) = "a:b"
+    $ "concat_with: ['a', 'b']) = 'a:b'"
+
+val () =
+    assert
+    $ concat_with (":", ["a", "b", "c"]) = "a:b:c"
+    $ "concat_with: ['a', 'b', 'c']) = 'a:b:c'"
+
+val () =
+    assert
+    $ concat_with (":", ["a"]) = "a"
+    $ "concat_with: ['a']) = 'a'"
+
+val () =
+    assert
+    $ quote_string "a" = "\"a\""
+    $ "quote_string: a = \"a\""
+
+val () =
+    assert
+    $ real_to_string_for_json 1.0 = "1.0"
+    $ "real_to_string_for_json: 1.0"
+
+val () =
+    assert
+    $ real_to_string_for_json ~1.0 = "-1.0"
+    $ "real_to_string_for_json: ~1.0"
+
+val () =
+    assert
+    $ json_to_string (String "a") = "\"a\""
+    $ "json_to_string: String a = a"
+
+
+val () =
+    assert
+    $ json_to_string (Array [String "a", String "b"]) = "[\"a\",\"b\"]"
+    $ "json_to_string: Array [String \"a\", String \"b\"]"
+
+
+val () =
+    assert
+    $ json_to_string False = "false"
+    $ "json_to_string: False"
+
+val () =
+    assert
+    $ json_to_string (Num 1.2) = "1.2"
+    $ "json_to_string: Num 1.2"
+
+val () =
+    assert
+    $ json_to_string (Object []) = "{}"
+    $ "json_to_string: Object []"
+
+val () = print (json_to_string (Object [("b", True), ("n", (Num 0.1))]))
+
+val () =
+    assert
+    $ json_to_string (Object [("b", True), ("n", (Num 0.1))]) = "{\"b\":true,\"n\":0.1}"
+    $ "json_to_string: Object [(\"b\", True), (\"n\", (Num 0.1))]"
+
+val () =
+    assert
+    $ assoc ("key", []) = NONE
+    $ "assoc: key []"
+
+val () =
+    assert
+    $ assoc ("key", [("key", "value")]) = SOME "value"
+    $ "assoc: key [(key, value)]"
+
+val () =
+    assert
+    $ count_occurrences ([], NotSorted) = []
+    $ "count_occurrences: [] = []"
+
+val () =
+    assert
+    $ count_occurrences (["a"], NotSorted) = [("a", 1)]
+    $ "count_occurrences: ['a'] = [('a', 1)]"
+
+val () =
+    assert
+    $ count_occurrences (["a", "b"], NotSorted) = [("a", 1), ("b", 1)]
+    $ "count_occurrences: ['a', 'b'] = [('a', 1), ('b', 1)]"
+
+val () =
+    assert
+    $ count_occurrences (["a", "a", "b"], NotSorted) = [("a", 2), ("b", 1)]
+    $ "count_occurrences: ['a', 'a', 'b'] = [('a', 2), ('b', 1)]"
+
+val () =
+    assert
+    $ count_occurrences (["b", "a", "a"], NotSorted) = [("b", 1), ("a", 2)]
+    $ "count_occurrences: ['b', 'a', 'a'] = [('b', 1), ('a', 2)]"
+
+val [] =
+    count_occurrences (["a", "a", "b", "a"], NotSorted)
+        handle NotSorted => 
+            assert true "count_occurrences: raises on unsorted list"
+            |> (fn _ => [])
+
+val () = complete ()

sml/week3_2020/list.sml

@@ -0,0 +1,32 @@
+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
+
+(* wrong definition, fold doesn't always returns array *)
+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

sml/week3_2020/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 >>

sml/week3_2020/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

sml/week4/hw3.sml

@@ -0,0 +1,123 @@
+use "operators.sml";
+use "list.sml";
+
+datatype pattern = 
+    Wildcard 
+    | Variable of string 
+    | UnitP 
+    | ConstP of int
+    | TupleP of pattern list 
+    | ConstructorP of string * pattern
+
+datatype valu = 
+    Const of int 
+    | Unit 
+    | Tuple of valu list 
+    | Constructor of string * valu
+
+exception NoAnswer
+
+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
+
+fun first_answer f lst =
+    case lst of
+       [] => raise NoAnswer
+     | head :: tail => 
+        case f head of
+        NONE => first_answer f tail
+        | SOME answer => answer
+
+fun all_answers 
+    (f: ('a -> 'b list option)) 
+    (lst: 'a list)
+    : 'b list option =
+    let
+        fun collect lst acc =
+            case lst of
+            [] => SOME acc
+            | head :: tail =>
+                case f head of
+                NONE => NONE
+                | SOME lst => collect tail (acc @ lst)
+    in
+        collect lst []
+    end
+
+fun sum a b = a + b
+(*  functions below are too similar to leave them as is.
+    Find a way how to extract reapiting code
+ *)
+fun count_wildcards (p: pattern) : int =
+    case p of
+    Wildcard => 1
+    | TupleP lst => foldl (count_wildcards >> sum) 0 lst
+    | ConstructorP (_, pattern) => count_wildcards pattern
+    | _ => 0
+ 
+fun collect_variables (p: pattern) : string list =
+    case p of
+    Variable str => [str]
+    | TupleP lst => foldl (collect_variables >> append) [] lst
+    | ConstructorP (_, pattern) => collect_variables pattern
+    | _ => []
+
+fun count_variable_lengths (p: pattern) : int =
+    p |> collect_variables |> foldl (size >> sum) 0
+
+fun count_wild_and_variable_lengths (p: pattern) : int =
+    count_wildcards p + count_variable_lengths p
+
+fun count_some_var (var, pattern) =
+    case pattern of
+       Variable v => if v = var then 1 else 0
+    | TupleP lst => foldl (fn p => fn acc => acc + count_some_var (var, p)) 0 lst
+    | ConstructorP (_, pattern) => count_some_var (var, pattern)
+    | _ => 0
+
+fun check_pat (p: pattern) : bool =
+    p 
+    |> collect_variables 
+    |> sort bigger_or_equal
+    |> distinct
+
+fun match (v: valu, p: pattern) : (string * valu) list option = 
+    case (p, v) of
+    (Wildcard, _) => SOME []
+    | (Variable a, v) => SOME [(a, v)]
+    | (UnitP, Unit) => SOME []
+    | (ConstP c1, Const c2) => if c1 = c2 then SOME [] else NONE
+    | (TupleP ps, Tuple vs) => (vs, ps) |> ListPair.zip |> all_answers match
+    | (ConstructorP (s1, p), Constructor (s2, v)) =>
+        if s1 = s2 then match (v, p) else NONE
+    | _ => NONE
+    
+fun first_match 
+    (v: valu) 
+    (patterns: pattern list) : 
+    (string * valu) list option =
+    SOME $ first_answer (fn p => match (v, p)) patterns
+        handle NoAnswer => NONE

sml/week4/hw3_test.sml

@@ -0,0 +1,197 @@
+use "test.sml";
+use "hw3.sml";
+
+fun comp a b = a <= b
+
+val () =
+    assert
+    $ sort comp [1, 2] = [1, 2]
+    $ "sort: sorting"
+
+val () =
+    assert
+    $ sort comp [1, 2, 1, 0, 4, 5, 2] = [0, 1, 1, 2, 2, 4, 5]
+    $ "sort: sorting x2"
+
+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 NONE =
+    first_answer (fn elm => NONE) []
+        handle NoAnswer =>
+            assert true "first_answer: rises on on empty lst" 
+            |> (fn _ => NONE)
+
+val NONE =
+    first_answer (fn elm => NONE) ["elm"]
+        handle NoAnswer =>
+            assert true "first_answer: rises on miss" 
+            |> (fn _ => NONE)
+
+val () =
+    assert
+    $ first_answer (fn elm => SOME elm) ["elm"] = "elm"
+    $ "first_answer: returns first answer"
+
+
+val () =
+    assert
+    $ first_answer 
+        (fn elm => if elm = "second" then SOME elm else NONE) 
+        ["elm", "second"] = "second"
+    $ "first_answer: returns some answer"
+
+val () =
+    assert
+    $ all_answers (fn elm => SOME elm) [] = SOME []
+    $ "all_answers: returns SOME [] on []"
+
+val () =
+    assert
+    $ all_answers (fn elm => SOME [elm]) ["a", "b"] = SOME ["a", "b"]
+    $ "all_answers: returns all answers"
+
+val () =
+    assert
+    $ all_answers 
+        (fn elm => if elm = "a" then SOME [elm] else NONE) 
+        ["a", "b"] = NONE
+    $ "all_answers: NONE on at least one NONE"
+
+val () =
+    assert
+    $ count_wildcards Wildcard = 1
+    $ "count_wildcards: Wildcard"
+
+val () =
+    assert
+    $ count_wildcards (TupleP [Wildcard, Wildcard]) = 2
+    $ "count_wildcards: TupleP"
+
+val () =
+    assert
+    $ count_wildcards (ConstructorP ("wild", Wildcard)) = 1
+    $ "count_wildcards: ConstructorP"
+
+val () =
+    assert
+    $ count_wild_and_variable_lengths (TupleP [Wildcard, Variable "var"]) = 4
+    $ "count_wild_and_variable_lengths: correct length"
+
+val () =
+    let
+      val input = (
+          "var", 
+          TupleP [
+              Wildcard, 
+              Variable "var", 
+              ConstructorP (
+                  "cons", 
+                  Variable "var")])
+        val expected = 2 
+    in
+    assert
+    $ count_some_var input = expected
+    $ "count_some_var: count vars"
+    end
+
+val () =
+    assert
+    $ check_pat (TupleP [Variable "a", Variable "b"]) = true
+    $ "check_pat: true on distinct"
+
+val () =
+    assert
+    $ check_pat (TupleP [Variable "a",ConstructorP ("cons", Variable "a")]) = false
+    $ "check_pat: false on same"
+
+val () =
+    assert
+    $ match (Unit, Wildcard) = SOME []
+    $ "match: SOME [] on wildcard"
+
+val () =
+    assert
+    $ match (Unit, Variable "var") = SOME [("var", Unit)]
+    $ "match: match with var"
+
+val () =
+    assert
+    $ match (Unit, UnitP) = SOME []
+    $ "match: match with unit"
+
+val () =
+    assert
+    $ match (Const 3, ConstP 3) = SOME []
+    $ "match: match with const"
+
+val () =
+    assert
+    $ match (Tuple [Const 3], TupleP [Variable "var"]) = SOME [("var", Const 3)]
+    $ "match: match with tuple"
+
+val () =
+    assert
+    $ match (Tuple [Const 3], TupleP [Variable "var"]) = SOME [("var", Const 3)]
+    $ "match: match with tuple"
+
+val () =
+    let
+      val arg = (Constructor ("c", Unit), ConstructorP ("c", Variable "var"))
+      val expect = SOME [("var", Unit)]
+    in
+        assert
+        $ match arg = expect
+        $ "match: match with Constructor"
+    end
+
+val () =
+    assert
+    $ first_match Unit [ConstP 17, Variable "v"] = SOME [("v", Unit)]
+    $ "first_match: finds match"
+
+val () =
+    assert
+    $ first_match Unit [ConstP 17] = NONE
+    $ "first_match: handles exeption"
+
+val () = complete ()

sml/week4/list.sml

@@ -0,0 +1,60 @@
+use "operators.sml";
+
+fun cons head tail = head :: tail
+
+fun append a b = a @ b
+
+fun foldl f acc lst =
+    case lst of
+    [] => acc
+    | head :: tail => foldl f (f head acc) tail
+
+fun reverse lst = foldl cons [] lst
+
+fun sort' f lst =
+    case lst of
+    [] => []
+    | [one] => [one]
+    | first :: second :: rest =>
+        if f first second
+        then first :: sort' f (second :: rest)
+        else second :: sort' f (first :: rest)
+
+(* only for sorted lists *)
+fun distinct lst =
+    case lst of
+    [] => true
+    | [_] => true
+    | first :: second :: rest =>
+        if first = second
+        then false
+        else distinct (second :: rest)
+
+(* simple recursive convergence *)
+fun fix f g x = 
+    if f g x = x 
+    then x 
+    else fix f g (f g x)
+
+(* naive bubble sort *)
+fun sort f x = fix sort' f x
+
+fun foldr f acc lst = lst |> reverse |> foldl f acc     
+
+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

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 >>

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