[lexer,parser,interpreter] Remove elif from the language.

While doing some research on how `case` and `cond` could be implemented, I noticed that Elixir doesn't have a singular `else if` construct. I can't be sure of the exact reason for this, but at the same time, I noticed that any use of an `else if` construct could easily be replaced with a `cond`, and would actually be shorter and more legible.

Because of that, I'm deciding to remove the corresponding `elif` construct from Myst, under the assumption that either `case` or `cond` will be implemented next.

Author: faultyserver

README.md

@@ -24,7 +24,7 @@ The following is a roadmap for a `v0.1.0` of Myst. It is _not necessarily_ compr
   - [X] strings
   - [X] symbols
 - [ ] Flow control and dependency loading
-  - [X] `if`/`unless`/`elif`/`else`
+  - [X] `if`/`unless`/`else`
   - [X] `while`/`until`
   - [ ] `break`
   - [ ] `return`

SYNTAX.md

@@ -35,7 +35,6 @@ begin
 break
 continue
 def
-elif
 else
 end
 ensure
@@ -270,10 +269,8 @@ Myst's flow control structures are almost directly taken from Ruby. However, the
 # conditions as blocks
 if a
   # only if `a` is truthy
-elif b
-  # only if `a` is falsey and `b` is truthy
 else
-  # only if neither of the above runs
+  # only if the above does not run
 end
 unless b
   # only if `b` is falsey

examples/conditionals.mt

@@ -15,20 +15,20 @@ def fib(n)
   end
 end
 
-puts(fib(10))
+IO.puts(fib(10))
 
 unless 1 == 2
-  puts("1 is not 2")
+  IO.puts("1 is not 2")
 end
 
 x = 5
 
 until x == 0
-  puts(x)
+  IO.puts(x)
   x = x - 1
 end
 
 while fib(x) < 10
-  puts(fib(x))
+  IO.puts(fib(x))
   x = x + 1
 end

spec/syntax/elif_expression_spec.cr

@@ -56,59 +56,7 @@ describe "Else Expression" do
       assert_invalid %q(
         if false
         else
-        elif wrong
-        end
-      )
-    end
-  end
-
-
-  describe "when following an `elif` block" do
-    it "is valid with an empty body" do
-      assert_valid %q(
-        if false
-        elif false
-        else
-        end
-      )
-    end
-
-    it "is valid with a single-expression body" do
-      assert_valid %q(
-        if true
-        elif true
-        else
-          1 + 1
-        end
-      )
-    end
-
-    it "is valid with a multi-expression body" do
-      assert_valid %q(
-        if true
-        elif true
-        else
-          a = 1
-          b = 1
-          a + b
-        end
-      )
-    end
-
-    it "allows a single expression immediately following the `else`" do
-      assert_valid %q(
         if false
-        elif false
-        else wrong end
-      )
-    end
-
-    it "is invalid with a succeeding conditional block" do
-      assert_invalid %q(
-        if false
-        elif wrong
-        else
-        elif wrong
         end
       )
     end
@@ -155,7 +103,7 @@ describe "Else Expression" do
       assert_invalid %q(
         unless false
         else
-        elif wrong
+        if wrong
         end
       )
     end

spec/syntax/else_expression_spec.cr

@@ -15,7 +15,6 @@ describe "Tokenizer" do
     "do"      => Token::Type::DO,
     "if"      => Token::Type::IF,
     "unless"  => Token::Type::UNLESS,
-    "elif"    => Token::Type::ELIF,
     "else"    => Token::Type::ELSE,
     "while"   => Token::Type::WHILE,
     "until"   => Token::Type::UNTIL,

spec/syntax/keyword_spec.cr

@@ -120,11 +120,6 @@ module Myst
       body        : Block,
       alternative : Node?
 
-    ast_node ElifExpression,
-      condition   : Node,
-      body        : Block,
-      alternative : Node?
-
     ast_node ElseExpression,
       body        : Block
 

src/myst/ast.cr

@@ -1,5 +1,5 @@
 class Myst::Interpreter
-  def visit(node : AST::IfExpression | AST::ElifExpression)
+  def visit(node : AST::IfExpression)
     recurse(node.condition.not_nil!)
     if stack.pop().truthy?
       recurse(node.body)

src/myst/interpreter/visitors/conditionals.cr

@@ -271,14 +271,14 @@ module Myst
       when Token::Type::IF
         advance
         condition = parse_expression
-        body = parse_block([Token::Type::ELSE, Token::Type::ELIF])
+        body = parse_block([Token::Type::ELSE])
         alternative = parse_conditional_alternative
         expect(Token::Type::END)
         return AST::IfExpression.new(condition, body, alternative)
       when Token::Type::UNLESS
         advance
         condition = parse_expression
-        body = parse_block([Token::Type::ELSE, Token::Type::ELIF])
+        body = parse_block([Token::Type::ELSE])
         alternative = parse_conditional_alternative
         expect(Token::Type::END)
         return AST::UnlessExpression.new(condition, body, alternative)
@@ -289,20 +289,14 @@ module Myst
 
     def parse_conditional_alternative
       case current_token.type
-      when Token::Type::ELIF
-        advance
-        condition = parse_expression
-        body = parse_block([Token::Type::ELSE, Token::Type::ELIF])
-        alternative = parse_conditional_alternative
-        return AST::ElifExpression.new(condition, body, alternative)
       when Token::Type::ELSE
         advance
         body = parse_block
         return AST::ElseExpression.new(body)
       when Token::Type::END
         return nil
       else
-        raise ParseError.new(current_token, [Token::Type::ELSE, Token::Type::ELIF, Token::Type::END])
+        raise ParseError.new(current_token, [Token::Type::ELSE, Token::Type::END])
       end
     end
 

src/myst/parser.cr

@@ -16,7 +16,6 @@ module Myst
       DO            # do
       IF            # if
       UNLESS        # unless
-      ELIF          # elif
       ELSE          # else
       WHILE         # while
       UNTIL         # until
@@ -68,7 +67,7 @@ module Myst
 
 
       def self.keywords
-        [REQUIRE, INCLUDE, MODULE, DEF, DO, IF, UNLESS, ELIF, ELSE, END, WHILE, UNTIL, TRUE, FALSE]
+        [REQUIRE, INCLUDE, MODULE, DEF, DO, IF, UNLESS, ELSE, END, WHILE, UNTIL, TRUE, FALSE]
       end
 
 
@@ -81,7 +80,6 @@ module Myst
           "do" => DO,
           "if" => IF,
           "unless" => UNLESS,
-          "elif" => ELIF,
           "else" => ELSE,
           "end" => END,
           "while" => WHILE,

src/myst/parser/token.cr

@@ -54,7 +54,7 @@ module Myst
       recurse [node.pattern, node.value]
     end
 
-    visit AST::IfExpression, AST::UnlessExpression, AST::ElifExpression do
+    visit AST::IfExpression, AST::UnlessExpression do
       io << "#{node.type_name}\n".colorize(:blue)
       recurse [node.condition, node.body, node.alternative].compact
     end