HCrisp/src/step2_eval.cr

#! /usr/bin/env crystal run

require "./readline"
require "./reader"
require "./printer"
require "./types"

# Note:
# Employed downcase names because Crystal prohibits uppercase names for methods

def eval_error(msg)
  raise Mal::EvalException.new msg
end

def num_func(func)
  -> (args : Array(Mal::Type)) {
    x, y = args[0].unwrap, args[1].unwrap
    eval_error "invalid arguments" unless x.is_a?(Int32) && y.is_a?(Int32)
    Mal::Type.new func.call(x, y)
  }
end

$repl_env = {
  "+" => num_func(->(x : Int32, y : Int32){ x + y }),
  "-" => num_func(->(x : Int32, y : Int32){ x - y }),
  "*" => num_func(->(x : Int32, y : Int32){ x * y }),
  "/" => num_func(->(x : Int32, y : Int32){ x / y }),
} of String => Mal::Func

def eval_ast(a, env)
  return a.map{|n| eval(n, env) as Mal::Type} if a.is_a? Mal::List
  return a unless a

  ast = a.unwrap
  case ast
  when Mal::Symbol
    if env.has_key? ast.str
      env[ast.str]
    else
      eval_error "'#{ast.str}' not found"
    end
  when Mal::List
    ast.each_with_object(Mal::List.new){|n, l| l << eval(n, env)}
  when Mal::Vector
    ast.each_with_object(Mal::Vector.new){|n, l| l << eval(n, env)}
  when Mal::HashMap
    ast.each{|k, v| ast[k] = eval(v, env)}
  else
    ast
  end
end

def read(str)
  read_str str
end

def eval(t, env)
  Mal::Type.new case ast = t.unwrap
  when Mal::List
    eval_error "empty list" if ast.empty?

    f = eval_ast(ast.first, env)
    ast.shift(1)
    args = eval_ast(ast, env)

    if f.is_a?(Mal::Func)
      f.call(args)
    else
      eval_error "expected function symbol as the first symbol of list"
    end
  else
    eval_ast(t, env)
  end
end

def print(result)
  pr_str(result, true)
end

def rep(str)
  print(eval(read(str), $repl_env))
end

while line = my_readline("user> ")
  begin
    puts rep(line)
  rescue e
    STDERR.puts e
  end
end