Introduce Prompt#display and don't color ouput for now

src/compiler/crystal/interpreter/interpreter.cr

@@ -1298,24 +1298,15 @@ class Crystal::Repl::Interpreter
           end
         end
 
-        value = interpreter.interpret(line_node, meta_vars, in_pry: true).to_s
+        value = interpreter.interpret(line_node, meta_vars, in_pry: true)
 
         # New local variables might have been declared during a pry session.
         # Remember them by asking them from the interpreter
         # (the interpreter will keep adding those, or migrate new ones
         # to their new type)
         local_vars = interpreter.local_vars
 
-        if @context.program.color?
-          begin
-            value = Crystal::SyntaxHighlighter::Colorize.highlight(value)
-          rescue
-            # Ignore highlight errors
-          end
-        end
-
-        print "=> "
-        puts value.to_s
+        prompt.display(value)
       rescue ex : EscapingException
         print "Unhandled exception: "
         print ex

src/compiler/crystal/interpreter/prompt.cr

@@ -0,0 +1,114 @@
+# Allows reading a prompt for the interpreter.
+class Crystal::Repl::Prompt
+  property line_number : Int32
+
+  def initialize(@context : Context, @show_nest : Bool)
+    @buffer = ""
+    @nest = 0
+    @incomplete = false
+    @line_number = 1
+  end
+
+  # Asks for a line of input, prefixed with the given prefix.
+  # Returns nil if the user pressed CTRL-C.
+  def prompt(prefix) : String?
+    prompt = String.build do |io|
+      io.print prefix
+      if @show_nest
+        io.print ':'
+        io.print @nest
+      end
+      io.print(@incomplete ? '*' : '>')
+      io.print ' '
+      if @nest == 0 && @incomplete
+        io.print "  "
+      else
+        io.print "  " * @nest if @nest > 0
+      end
+    end
+
+    print prompt
+    line = gets
+    return unless line
+
+    if @context.program.color?
+      # Go back one line to print it again colored
+      print "\033[F"
+      print prompt
+
+      colored_line = line
+      begin
+        colored_line = Crystal::SyntaxHighlighter::Colorize.highlight(colored_line)
+      rescue
+        # Ignore highlight errors
+      end
+
+      puts colored_line
+    end
+
+    new_buffer =
+      if @buffer.empty?
+        line
+      else
+        "#{@buffer}\n#{line}"
+      end
+
+    new_buffer
+  end
+
+  # Parses the given input with the given var_scopes.
+  # The input must be that returned from `#prompt`.
+  # Returns a parsed ASTNode if the input was valid Crystal syntax.
+  # If the input was partial Crystal syntax, `nil` is returned
+  # but the partial input is remembered. Next time `#prompt` is called,
+  # the returned value there will be the new complete input (what there
+  # was before plus the new input, separated by a new line).
+  def parse(input : String, var_scopes : Array(Set(String))) : ASTNode?
+    parser = Parser.new(
+      input,
+      string_pool: @context.program.string_pool,
+      var_scopes: var_scopes,
+    )
+
+    begin
+      node = parser.parse
+
+      @nest = 0
+      @buffer = ""
+      @line_number += 1
+      @incomplete = false
+
+      node
+    rescue ex : Crystal::SyntaxException
+      # TODO: improve this
+      case ex.message
+      when "unexpected token: EOF",
+           "expecting identifier 'end', not 'EOF'"
+        @nest = parser.type_nest + parser.def_nest + parser.fun_nest
+        @buffer = input
+        @line_number += 1
+        @incomplete = @nest == 0
+      when "expecting token ']', not 'EOF'",
+           "unterminated array literal",
+           "unterminated hash literal",
+           "unterminated tuple literal"
+        @nest = parser.type_nest + parser.def_nest + parser.fun_nest
+        @buffer = input
+        @line_number += 1
+        @incomplete = true
+      else
+        puts "Error: #{ex.message}"
+        @nest = 0
+        @buffer = ""
+        @incomplete = false
+      end
+      nil
+    end
+  end
+
+  # Displays a value, preceding it with "=> ".
+  def display(value : Value)
+    print "=> "
+    puts value
+  end
+end

src/compiler/crystal/interpreter/repl.cr

@@ -37,18 +37,8 @@ class Crystal::Repl
       next unless node
 
       begin
-        value = interpret(node).to_s
-
-        if @context.program.color?
-          begin
-            value = Crystal::SyntaxHighlighter::Colorize.highlight(value)
-          rescue
-            # Ignore highlight errors
-          end
-        end
-
-        print "=> "
-        puts value
+        value = interpret(node)
+        prompt.display(value)
       rescue ex : EscapingException
         @nest = 0
         @buffer = ""