[parser,ast] Allow function captures as block parameters for instantiation

spec/syntax/parser_spec.cr

@@ -529,19 +529,19 @@ describe "Parser" do
   it_does_not_parse %q(a.b! = 1)
   # Assigned Anonymous Functions called should be coerced to a Call
   it_parses %q(
-    foo = fn 
-            ->() { } 
+    foo = fn
+            ->() { }
           end
     foo()
   ), SimpleAssign.new(v("foo"), AnonymousFunction.new([Block.new])), Call.new(nil, "foo")
   it_parses %q(
-    foo = fn 
-            ->() { } 
+    foo = fn
+            ->() { }
           end
     bar = foo
     bar()
   ), SimpleAssign.new(v("foo"), AnonymousFunction.new([Block.new])), SimpleAssign.new(v("bar"), v("foo")), Call.new(nil, "bar")
-  
+
   # Assignments can not be made to literal values.
   it_does_not_parse %q(2 = 4),          /cannot assign to literal value/i
   it_does_not_parse %q(2.56 = 4),       /cannot assign to literal value/i
@@ -1126,6 +1126,13 @@ describe "Parser" do
   it_parses %q(
     %Thing{ } { |a,b| }
   ),                      Instantiation.new(c("Thing"), block: Block.new([p("a"), p("b")]))
+  # The block parameter can also be specified as a capture
+  it_parses %q(%Thing{&block}), Instantiation.new(c("Thing"), block: FunctionCapture.new(Call.new(nil, "block")))
+  it_parses %q(%Thing{
+    &fn
+      ->(){}
+    end
+  }),                    Instantiation.new(c("Thing"), block: FunctionCapture.new(AnonymousFunction.new([Block.new])))
 
   # Also as in a Call, trailing commas and the like are invalid
   it_does_not_parse %q(%Thing{ 1, })

src/myst/syntax/ast.cr

@@ -781,7 +781,7 @@ module Myst
   class Instantiation < Node
     property  type    : Node
     property  args    : Array(Node)
-    property! block   : Block?
+    property! block   : (Block | FunctionCapture)?
 
     def initialize(@type, @args=[] of Node, @block=nil)
     end

src/myst/syntax/parser.cr

@@ -724,8 +724,8 @@ module Myst
     def parse_var_or_call(receiver=nil)
       start = expect(Token::Type::IDENT, Token::Type::CONST)
       name  = start.value
-      
-      skip_space      
+
+      skip_space
       if receiver.nil? && current_token.type != Token::Type::LPAREN
         if name.starts_with?('_')
           return Underscore.new(name).at(start.location)
@@ -932,9 +932,15 @@ module Myst
       else
         loop do
           skip_space_and_newlines
-          inst.args << parse_expression
+          case current_token.type
+          when Token::Type::AMPERSAND
+            inst.block = parse_function_capture
+          else
+            inst.args << parse_expression
+          end
           skip_space_and_newlines
 
+
           # If there is no comma, this is the last argument, and a closing
           # parenthesis should be expected.
           unless accept(Token::Type::COMMA)
@@ -946,7 +952,15 @@ module Myst
       end
 
       skip_space
-      if inst.block = parse_optional_block
+      if inline_block = parse_optional_block
+        # If a block argument was already specified by a function capture,
+        # inline blocks are not allowed. Checking this after parsing the
+        # optional block allows for a more helpful error message.
+        if inst.block?
+          raise ParseError.new(current_location, "A block argument has already been given by a captured function. Defining an extra inline block is not allowed.")
+        end
+
+        inst.block = inline_block
         return inst.at_end(inst.block)
       end