add forth exercise

config.json

@@ -464,6 +464,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 8
+      },
+      {
+        "slug": "forth",
+        "name": "Forth",
+        "uuid": "a9f94a1f-2ac6-4792-b6e7-d2ea0d65549c",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 9
       }
     ]
   },

exercises/practice/forth/.docs/instructions.md

@@ -0,0 +1,23 @@
+# Instructions
+
+Implement an evaluator for a very simple subset of Forth.
+
+[Forth][forth]
+is a stack-based programming language.
+Implement a very basic evaluator for a small subset of Forth.
+
+Your evaluator has to support the following words:
+
+- `+`, `-`, `*`, `/` (integer arithmetic)
+- `DUP`, `DROP`, `SWAP`, `OVER` (stack manipulation)
+
+Your evaluator also has to support defining new words using the customary syntax: `: word-name definition ;`.
+
+To keep things simple the only data type you need to support is signed integers of at least 16 bits size.
+
+You should use the following rules for the syntax: a number is a sequence of one or more (ASCII) digits, a word is a sequence of one or more letters, digits, symbols or punctuation that is not a number.
+(Forth probably uses slightly different rules, but this is close enough.)
+
+Words are case-insensitive.
+
+[forth]: https://en.wikipedia.org/wiki/Forth_%28programming_language%29

exercises/practice/forth/.meta/Example.lean

@@ -0,0 +1,70 @@
+import Std
+
+namespace Forth
+
+abbrev Stack := List Int
+abbrev Op := Stack -> Except String Stack
+abbrev OpMap := Std.TreeMap String Op
+
+structure State where
+  stack : Stack
+  opMap : OpMap
+
+def applyUnary (op : Int -> Except String Stack) : Op
+  | [] => .error "empty stack"
+  | x :: xs => op x >>= fun rs => .ok (rs ++ xs)
+
+def applyBinary (op : Int -> Int -> Except String Stack) : Op
+  | [] => .error "empty stack"
+  | _ :: [] => .error "only one value on the stack"
+  | x1 :: x2 :: xs => op x2 x1 >>= fun rs => .ok (rs ++ xs)
+
+def parseExpression (state : State) (expression : String) : Except String State :=
+  match expression.toInt? with
+  | some value => .ok { state with stack := (value :: state.stack) } -- if number, adds it to the stack
+  | none => match state.opMap.get? expression with
+            | some op => op state.stack >>= (.ok { state with stack := · }) -- applies op, gets the resulting stack and replaces state stack with it
+            | none => .error "undefined operation"
+
+def checkOp (expression : String) (opMap : OpMap) : Op :=
+  match opMap.get? expression with
+  | some op => op  -- if expression is a key already in map, return its op -> this allows for indirect ops that reference other ops
+  | none => fun stack => parseExpression { stack := stack, opMap := opMap } expression >>= fun state => .ok state.stack -- otherwise, an op is nothing more than an expression to parse
+
+def addUserOp (pattern : String) (state : State) : Op :=
+  let ops := pattern.splitOn " " |> (·.map (checkOp · state.opMap))
+  ops.foldlM (fun acc op => op acc)
+
+def parseInstruction (state : State) (instruction : String) : Except String State :=
+  let expressions := instruction.toLower.splitOn " "
+  match expressions with
+  | [] => .ok state
+  | ":" :: key :: xs =>
+    match key.toInt? with
+    | some _ => .error "illegal operation"  -- number can't be an user-defined op
+    | none =>
+      let userOp := String.intercalate " " xs.dropLast -- drops ';' at the end
+      let op := addUserOp userOp state
+      .ok { state with opMap := state.opMap.insert key op } -- adds op to map, possibly replacing previous op with same key
+  | _ => expressions.foldlM parseExpression state -- parses expressions, short-circuiting on error
+
+def evaluate (instructions : List String) : Except String Stack :=
+  let initial : State := {
+    stack := [],
+    opMap := .ofList [
+      -- in all binary ops, b is the top of the stack and a is further inside
+      ("+", applyBinary (fun a b => .ok [a + b])),
+      ("-", applyBinary (fun a b => .ok [a - b])),
+      ("*", applyBinary (fun a b => .ok [a * b])),
+      ("/", applyBinary (fun a b => if b == 0 then .error "divide by zero" else .ok [a / b])),
+      ("dup", applyUnary (fun a => .ok [a, a])),
+      ("drop", applyUnary (fun a => .ok [])),
+      ("swap", applyBinary (fun a b => .ok [a, b])),
+      ("over", applyBinary (fun a b => .ok [a, b, a]))
+    ]
+  }
+
+  instructions.foldlM parseInstruction initial -- monadic fold, short-circuits on error
+  >>= fun state => .ok state.stack.reverse     -- if successful, returns reversed stack, lifted to Except
+
+end Forth

exercises/practice/forth/.meta/config.json

@@ -0,0 +1,17 @@
+{
+  "authors": [
+    "oxe-i"
+  ],
+  "files": {
+    "solution": [
+      "Forth.lean"
+    ],
+    "test": [
+      "ForthTest.lean"
+    ],
+    "example": [
+      ".meta/Example.lean"
+    ]
+  },
+  "blurb": "Implement an evaluator for a very simple subset of Forth."
+}

exercises/practice/forth/.meta/tests.toml

@@ -0,0 +1,176 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[9962203f-f00a-4a85-b404-8a8ecbcec09d]
+description = "parsing and numbers -> numbers just get pushed onto the stack"
+
+[fd7a8da2-6818-4203-a866-fed0714e7aa0]
+description = "parsing and numbers -> pushes negative numbers onto the stack"
+
+[9e69588e-a3d8-41a3-a371-ea02206c1e6e]
+description = "addition -> can add two numbers"
+
+[52336dd3-30da-4e5c-8523-bdf9a3427657]
+description = "addition -> errors if there is nothing on the stack"
+
+[06efb9a4-817a-435e-b509-06166993c1b8]
+description = "addition -> errors if there is only one value on the stack"
+
+[1e07a098-c5fa-4c66-97b2-3c81205dbc2f]
+description = "addition -> more than two values on the stack"
+
+[09687c99-7bbc-44af-8526-e402f997ccbf]
+description = "subtraction -> can subtract two numbers"
+
+[5d63eee2-1f7d-4538-b475-e27682ab8032]
+description = "subtraction -> errors if there is nothing on the stack"
+
+[b3cee1b2-9159-418a-b00d-a1bb3765c23b]
+description = "subtraction -> errors if there is only one value on the stack"
+
+[2c8cc5ed-da97-4cb1-8b98-fa7b526644f4]
+description = "subtraction -> more than two values on the stack"
+
+[5df0ceb5-922e-401f-974d-8287427dbf21]
+description = "multiplication -> can multiply two numbers"
+
+[9e004339-15ac-4063-8ec1-5720f4e75046]
+description = "multiplication -> errors if there is nothing on the stack"
+
+[8ba4b432-9f94-41e0-8fae-3b3712bd51b3]
+description = "multiplication -> errors if there is only one value on the stack"
+
+[5cd085b5-deb1-43cc-9c17-6b1c38bc9970]
+description = "multiplication -> more than two values on the stack"
+
+[e74c2204-b057-4cff-9aa9-31c7c97a93f5]
+description = "division -> can divide two numbers"
+
+[54f6711c-4b14-4bb0-98ad-d974a22c4620]
+description = "division -> performs integer division"
+
+[a5df3219-29b4-4d2f-b427-81f82f42a3f1]
+description = "division -> errors if dividing by zero"
+
+[1d5bb6b3-6749-4e02-8a79-b5d4d334cb8a]
+description = "division -> errors if there is nothing on the stack"
+
+[d5547f43-c2ff-4d5c-9cb0-2a4f6684c20d]
+description = "division -> errors if there is only one value on the stack"
+
+[f224f3e0-b6b6-4864-81de-9769ecefa03f]
+description = "division -> more than two values on the stack"
+
+[ee28d729-6692-4a30-b9be-0d830c52a68c]
+description = "combined arithmetic -> addition and subtraction"
+
+[40b197da-fa4b-4aca-a50b-f000d19422c1]
+description = "combined arithmetic -> multiplication and division"
+
+[f749b540-53aa-458e-87ec-a70797eddbcb]
+description = "combined arithmetic -> multiplication and addition"
+
+[c8e5a4c2-f9bf-4805-9a35-3c3314e4989a]
+description = "combined arithmetic -> addition and multiplication"
+
+[c5758235-6eef-4bf6-ab62-c878e50b9957]
+description = "dup -> copies a value on the stack"
+
+[f6889006-5a40-41e7-beb3-43b09e5a22f4]
+description = "dup -> copies the top value on the stack"
+
+[40b7569c-8401-4bd4-a30d-9adf70d11bc4]
+description = "dup -> errors if there is nothing on the stack"
+
+[1971da68-1df2-4569-927a-72bf5bb7263c]
+description = "drop -> removes the top value on the stack if it is the only one"
+
+[8929d9f2-4a78-4e0f-90ad-be1a0f313fd9]
+description = "drop -> removes the top value on the stack if it is not the only one"
+
+[6dd31873-6dd7-4cb8-9e90-7daa33ba045c]
+description = "drop -> errors if there is nothing on the stack"
+
+[3ee68e62-f98a-4cce-9e6c-8aae6c65a4e3]
+description = "swap -> swaps the top two values on the stack if they are the only ones"
+
+[8ce869d5-a503-44e4-ab55-1da36816ff1c]
+description = "swap -> swaps the top two values on the stack if they are not the only ones"
+
+[74ba5b2a-b028-4759-9176-c5c0e7b2b154]
+description = "swap -> errors if there is nothing on the stack"
+
+[dd52e154-5d0d-4a5c-9e5d-73eb36052bc8]
+description = "swap -> errors if there is only one value on the stack"
+
+[a2654074-ba68-4f93-b014-6b12693a8b50]
+description = "over -> copies the second element if there are only two"
+
+[c5b51097-741a-4da7-8736-5c93fa856339]
+description = "over -> copies the second element if there are more than two"
+
+[6e1703a6-5963-4a03-abba-02e77e3181fd]
+description = "over -> errors if there is nothing on the stack"
+
+[ee574dc4-ef71-46f6-8c6a-b4af3a10c45f]
+description = "over -> errors if there is only one value on the stack"
+
+[ed45cbbf-4dbf-4901-825b-54b20dbee53b]
+description = "user-defined words -> can consist of built-in words"
+
+[2726ea44-73e4-436b-bc2b-5ff0c6aa014b]
+description = "user-defined words -> execute in the right order"
+
+[9e53c2d0-b8ef-4ad8-b2c9-a559b421eb33]
+description = "user-defined words -> can override other user-defined words"
+
+[669db3f3-5bd6-4be0-83d1-618cd6e4984b]
+description = "user-defined words -> can override built-in words"
+
+[588de2f0-c56e-4c68-be0b-0bb1e603c500]
+description = "user-defined words -> can override built-in operators"
+
+[ac12aaaf-26c6-4a10-8b3c-1c958fa2914c]
+description = "user-defined words -> can use different words with the same name"
+
+[53f82ef0-2750-4ccb-ac04-5d8c1aefabb1]
+description = "user-defined words -> can define word that uses word with the same name"
+
+[35958cee-a976-4a0f-9378-f678518fa322]
+description = "user-defined words -> cannot redefine non-negative numbers"
+
+[df5b2815-3843-4f55-b16c-c3ed507292a7]
+description = "user-defined words -> cannot redefine negative numbers"
+
+[5180f261-89dd-491e-b230-62737e09806f]
+description = "user-defined words -> errors if executing a non-existent word"
+
+[3c8bfef3-edbb-49c1-9993-21d4030043cb]
+description = "user-defined words -> only defines locally"
+include = false
+
+[7b83bb2e-b0e8-461f-ad3b-96ee2e111ed6]
+description = "case-insensitivity -> DUP is case-insensitive"
+
+[339ed30b-f5b4-47ff-ab1c-67591a9cd336]
+description = "case-insensitivity -> DROP is case-insensitive"
+
+[ee1af31e-1355-4b1b-bb95-f9d0b2961b87]
+description = "case-insensitivity -> SWAP is case-insensitive"
+
+[acdc3a49-14c8-4cc2-945d-11edee6408fa]
+description = "case-insensitivity -> OVER is case-insensitive"
+
+[5934454f-a24f-4efc-9fdd-5794e5f0c23c]
+description = "case-insensitivity -> user-defined words are case-insensitive"
+
+[037d4299-195f-4be7-a46d-f07ca6280a06]
+description = "case-insensitivity -> definitions are case-insensitive"

exercises/practice/forth/Forth.lean

@@ -0,0 +1,6 @@
+namespace Forth
+
+def evaluate (instructions : List String) : Except String (List Int) :=
+  sorry
+
+end Forth