Implement an evaluator for a very simple subset of 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.
Follow the setup instructions for Crystal here:
http://exercism.io/languages/crystal
More help installing can be found here:
http://crystal-lang.org/docs/installation/index.html
Execute the tests with:
$ crystal spec
In each test suite all but the first test have been skipped.
Once you get a test passing, you can unskip the next one by changing pending
to it
.
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
require "spec"
require "../src/*"
describe "Forth" do
it "numbers just get pushed onto the stack" do
Forth.evaluate("1 2 3 4 5").should eq([1, 2, 3, 4, 5])
end
pending "can add two numbers" do
Forth.evaluate("1 2 +").should eq([3])
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("+")
end
end
pending "errors if there is only one value on the stack" do
expect_raises(Exception) do
Forth.evaluate("1 +")
end
end
pending "can subtract two numbers" do
Forth.evaluate("3 4 -").should eq([-1])
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("-")
end
end
pending "errors if there is only one value on the stack" do
expect_raises(Exception) do
Forth.evaluate("1 -")
end
end
pending "can multiply two numbers" do
Forth.evaluate("2 4 *").should eq([8])
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("*")
end
end
pending "errors if there is only one value on the stack" do
expect_raises(Exception) do
Forth.evaluate("1 *")
end
end
pending "can divide two numbers" do
Forth.evaluate("12 3 /").should eq([4])
end
pending "performs integer division" do
Forth.evaluate("8 3 /").should eq([2])
end
pending "errors if dividing by zero" do
expect_raises(Exception) do
Forth.evaluate("4 0 /")
end
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("/")
end
end
pending "errors if there is only one value on the stack" do
expect_raises(Exception) do
Forth.evaluate("1 /")
end
end
pending "addition and subtraction" do
Forth.evaluate("1 2 + 4 -").should eq([-1])
end
pending "multiplication and division" do
Forth.evaluate("2 4 * 3 /").should eq([2])
end
pending "copies a value on the stack" do
Forth.evaluate("1 dup").should eq([1, 1])
end
pending "copies the top value on the stack" do
Forth.evaluate("1 2 dup").should eq([1, 2, 2])
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("dup")
end
end
pending "removes the top value on the stack if it is the only one" do
Forth.evaluate("1 drop").should eq([] of Int32)
end
pending "removes the top value on the stack if it is not the only one" do
Forth.evaluate("1 2 drop").should eq([1])
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("drop")
end
end
pending "swaps the top two values on the stack if they are the only ones" do
Forth.evaluate("1 2 swap").should eq([2, 1])
end
pending "swaps the top two values on the stack if they are not the only ones" do
Forth.evaluate("1 2 3 swap").should eq([1, 3, 2])
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("swap")
end
end
pending "errors if there is only one value on the stack" do
expect_raises(Exception) do
Forth.evaluate("1 swap")
end
end
pending "copies the second element if there are only two" do
Forth.evaluate("1 2 over").should eq([1, 2, 1])
end
pending "copies the second element if there are more than two" do
Forth.evaluate("1 2 3 over").should eq([1, 2, 3, 2])
end
pending "errors if there is nothing on the stack" do
expect_raises(Exception) do
Forth.evaluate("over")
end
end
pending "errors if there is only one value on the stack" do
expect_raises(Exception) do
Forth.evaluate("1 over")
end
end
pending "can consist of built-in words" do
Forth.evaluate(": dup-twice dup dup ;1 dup-twice").should eq([1, 1, 1])
end
pending "execute in the right order" do
Forth.evaluate(": countup 1 2 3 ;countup").should eq([1, 2, 3])
end
pending "can override other user-defined words" do
Forth.evaluate(": foo dup ;: foo dup dup ;1 foo").should eq([1, 1, 1])
end
pending "can override built-in words" do
Forth.evaluate(": swap dup ;1 swap").should eq([1, 1])
end
pending "can override built-in operators" do
Forth.evaluate(": + * ;3 4 +").should eq([12])
end
pending "cannot redefine numbers" do
expect_raises(Exception) do
Forth.evaluate(": 1 2 ;")
end
end
pending "errors if executing a non-existent word" do
expect_raises(Exception) do
Forth.evaluate("foo")
end
end
pending "DUP is case-insensitive" do
Forth.evaluate("1 DUP Dup dup").should eq([1, 1, 1, 1])
end
pending "DROP is case-insensitive" do
Forth.evaluate("1 2 3 4 DROP Drop drop").should eq([1])
end
pending "SWAP is case-insensitive" do
Forth.evaluate("1 2 SWAP 3 Swap 4 swap").should eq([2, 3, 4, 1])
end
pending "OVER is case-insensitive" do
Forth.evaluate("1 2 OVER Over over").should eq([1, 2, 1, 2, 1])
end
pending "user-defined words are case-insensitive" do
Forth.evaluate(": foo dup ;1 FOO Foo foo").should eq([1, 1, 1, 1])
end
pending "definitions are case-insensitive" do
Forth.evaluate(": SWAP DUP Dup dup ;1 swap").should eq([1, 1, 1, 1])
end
end
require "string_scanner"
require "./stack"
class Forth
PARSING_ITER_LIMIT = 1000
ASSIGNMENT_REGEX = /\:(.*?)\;/
NON_WORD_REGEX = /\s|$/
NUMBER_REGEX = /^\d+$/
getter stack = Stack.new
getter assignments = {} of String => Array(String)
def self.evaluate(instructions : String)
new(instructions).evaluate
end
def initialize(@instructions : String); end
def evaluate
scanner = StringScanner.new(@instructions)
PARSING_ITER_LIMIT.times do |i|
break if scanner.eos?
if assignment = scanner.scan(ASSIGNMENT_REGEX)
process_assignment(assignment)
next
end
word = scanner.scan_until(NON_WORD_REGEX)
if word
process_word(word.strip)
else
raise "Unexpected Nil word occurred"
end
raise "Parsing reached a hard limit" if i + 1 == PARSING_ITER_LIMIT
end
stack.to_a
end
def process_assignment(assignment)
re_match = assignment.match(ASSIGNMENT_REGEX)
unless re_match && re_match[1]?
raise "Assigment failure, invalid format: #{assignment}"
end
pieces = re_match[1].strip.split(" ")
name = pieces[0].downcase
if name =~ NUMBER_REGEX
raise "Assignment cannot redefine numbers: #{assignment}"
end
body = pieces[1..-1].map(&.downcase)
assignments[name] = body
end
def process_word(word)
word = word.downcase
case word_type(word)
when :custom
assignments[word].each { |w| process_word(w) }
when :numeric
stack.push(word.to_i)
when :add
stack.pop_n_push(2) { |vals| vals[0] + vals[1] }
when :subtract
stack.pop_n_push(2) { |vals| vals[0] - vals[1] }
when :multiply
stack.pop_n_push(2) { |vals| vals[0] * vals[1] }
when :divide
stack.pop_n_push(2) { |vals| vals[0] / vals[1] }
when :duplicate
val = stack.pop[0]
stack.push(val, val)
when :drop
stack.pop
when :swap
a, b = stack.pop(2)
stack.push(b, a)
when :over
a, b = stack.pop(2)
stack.push(a, b, a)
else
raise "Unknown word: '#{word}'"
end
end
def word_type(word)
case word
when custom_definition
:custom
when NUMBER_REGEX
:numeric
when "+"
:add
when "-"
:subtract
when "*"
:multiply
when "/"
:divide
when "dup"
:duplicate
when "drop"
:drop
when "swap"
:swap
when "over"
:over
end
end
def custom_definition
->(word : String) { assignments.has_key?(word) }
end
end
class Stack
@array = [] of Int32
def pop(n = 1)
vals = @array.pop(n)
raise IndexError.new if vals.size < n
vals
end
def push(*vals)
@array.push(*vals)
end
def pop_n_push(n = 1, &block)
push(yield(pop(n)))
end
def to_a
@array
end
end
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I really liked solving this one! ðŸ¥³