diff --git a/README.md b/README.md
index f07f9e6..9267eaa 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ Please see [mal test cases](https://github.com/rhysd/Crisp/tree/master/spec/cris
 ## Development Environment
 
 - OS X
-- Crystal 0.7.1 ~ 0.7.3 or 0.7.7 ~ 0.11.0 or 0.17.4 or 0.18.4
+- Crystal 0.23.1
 
 ## License
 
diff --git a/src/crisp/core.cr b/src/crisp/core.cr
index e21b626..7e9dbf9 100644
--- a/src/crisp/core.cr
+++ b/src/crisp/core.cr
@@ -34,7 +34,7 @@ module Crisp
     a = args.first.unwrap
     case a
     when Array
-      a.size as Int32
+      a.size.as Int32
     when Nil
       0
     else
@@ -77,7 +77,7 @@ module Crisp
   end
 
   def cons(args)
-    head, tail = args[0] as Crisp::Expr, args[1].unwrap
+    head, tail = args[0].as Crisp::Expr, args[1].unwrap
     Crisp.eval_error "2nd arg of cons must be list" unless tail.is_a? Array
     ([head] + tail).to_crisp_value
   end
diff --git a/src/crisp/evaluator.cr b/src/crisp/evaluator.cr
index c2aafb5..bcd1f68 100644
--- a/src/crisp/evaluator.cr
+++ b/src/crisp/evaluator.cr
@@ -15,11 +15,11 @@ module Crisp
       -> (args : Array(Crisp::Expr)) {
       new_env = Crisp::Env.new(env, binds, args)
       eval(body, new_env)
-      } as Crisp::Func
+      }.as Crisp::Func
     end
 
     def eval_ast(ast, env)
-      return ast.map{|n| eval(n, env) as Crisp::Expr} if ast.is_a? Array
+      return ast.map{|n| eval(n, env).as Crisp::Expr} if ast.is_a? Array
 
       val = ast.unwrap
 
@@ -94,8 +94,8 @@ module Crisp
       while macro_call?(ast, env)
 
         # Already checked in macro_call?
-        list = ast.unwrap as Crisp::List
-        func_sym = list[0].unwrap as Crisp::Symbol
+        list = ast.unwrap.as Crisp::List
+        func_sym = list[0].unwrap.as Crisp::Symbol
         func = env.get(func_sym.str).unwrap
 
         case func
@@ -113,7 +113,7 @@ module Crisp
 
     macro invoke_list(l, env)
       f = eval({{l}}.first, {{env}}).unwrap
-      args = eval_ast({{l}}[1..-1], {{env}}) as Array
+      args = eval_ast({{l}}[1..-1], {{env}}).as Array
 
       case f
       when Crisp::Closure
diff --git a/src/crisp/expr.cr b/src/crisp/expr.cr
index eccf59b..9be1dfb 100644
--- a/src/crisp/expr.cr
+++ b/src/crisp/expr.cr
@@ -13,6 +13,8 @@ module Crisp
     end
   end
 
+  class Expr; end
+
   class List < Array(Expr)
   end
 
@@ -34,7 +36,7 @@ module Crisp
 
   class Closure
     property :ast, :params, :env, :fn
-    def initialize(@ast : Expr, @params : List | Vector, @env : Env, @fn : Func)
+    def initialize(@ast : Expr, @params : Array(Crisp::Expr), @env : Env, @fn : Func)
     end
   end
 
@@ -46,7 +48,7 @@ module Crisp
 
     def initialize(@val : Type)
       @is_macro = false
-      @meta = nil as Expr?
+      @meta = nil.as Expr?
     end
 
     def initialize(other : Expr)
diff --git a/src/crisp/printer.cr b/src/crisp/printer.cr
index 15311d5..88ba11b 100644
--- a/src/crisp/printer.cr
+++ b/src/crisp/printer.cr
@@ -10,8 +10,8 @@ module Crisp
       when Nil          then "nil"
       when Bool         then value.to_s
       when Int32        then value.to_s
-      when Crisp::List    then "(#{value.map{|v| print(v) as String}.join(" ")})"
-      when Crisp::Vector  then "[#{value.map{|v| print(v) as String}.join(" ")}]"
+      when Crisp::List    then "(#{value.map{|v| print(v).as String}.join(" ")})"
+      when Crisp::Vector  then "[#{value.map{|v| print(v).as String}.join(" ")}]"
       when Crisp::Symbol  then value.str.to_s
       when Crisp::Func    then "<function>"
       when Crisp::Closure then "<closure>"