Chunk of work on if-expr AST

don't expect this to compile yet

TODO.md

@@ -1,6 +1,7 @@
 # TODO items
 
 ## General code cleanup
+- I think I can restructure the parser to get rid of most instances of expect!, at least at the beginning of a rule
 DONE -experiment with storing metadata via ItemIds on AST nodes (cf. https://rust-lang.github.io/rustc-guide/hir.html, https://github.com/rust-lang/rust/blob/master/src/librustc/hir/mod.rs )
 -implement and test open/use statements
 -implement field access

schala-lang/language/src/ast.rs

@@ -189,8 +189,8 @@ pub enum ExpressionKind {
     indexers: Vec<Expression>,
   },
   IfExpression {
-    discriminator: Box<Discriminator>,
-    body: Box<IfExpressionBody>,
+    discriminator: Option<Box<Expression>>,
+    body: IfExpressionBody,
   },
   WhileExpression {
     condition: Option<Box<Expression>>,
@@ -218,36 +218,33 @@ pub enum InvocationArgument {
   Ignored
 }
 
-#[derive(Debug, PartialEq, Clone)]
-pub enum Discriminator {
-  Simple(Expression),
-  BinOp(Expression, BinOp)
-}
-
 #[derive(Debug, PartialEq, Clone)]
 pub enum IfExpressionBody {
-  SimpleConditional(Block, Option<Block>),
-  SimplePatternMatch(Pattern, Block, Option<Block>),
-  GuardList(Vec<GuardArm>)
+  SimpleConditional {
+    then_case: Block,
+    else_case: Option<Block>
+  },
+  SimplePatternMatch {
+    pattern: Pattern,
+    then_case: Block,
+    else_case: Option<Block>
+  },
+  CondList(Vec<ConditionArm>)
 }
 
 #[derive(Debug, PartialEq, Clone)]
-pub struct GuardArm {
-  pub guard: Guard,
+pub struct ConditionArm {
+  pub condition: Condition,
+  pub guard: Option<Expression>,
   pub body: Block,
 }
 
 #[derive(Debug, PartialEq, Clone)]
-pub enum Guard {
-  None,
-  Pat(Pattern),
-  HalfExpr(HalfExpr)
-}
-
-#[derive(Debug, PartialEq, Clone)]
-pub struct HalfExpr {
-  pub op: Option<BinOp>,
-  pub expr: ExpressionKind,
+pub enum Condition {
+  Pattern(Pattern),
+  TruncatedOp(BinOp, Box<Expression>),
+  Expression(Box<Expression>),
+  Else,
 }
 
 #[derive(Debug, PartialEq, Clone)]

schala-lang/language/src/parsing.rs

@@ -119,13 +119,14 @@
 //! discriminator
 //!
 //! ```text
-//! if_expr := "if" discriminator ("then" simple_conditional | "is" simple_pattern_match | cond_block)
+//! if_expr := "if" discriminator if_expr_body
+//! if_expr_body := ("then" simple_conditional | "is" simple_pattern_match | cond_block)
 //! discriminator := ε | expression
-//! simple_conditional := expr_or_block else_clause
+//! simple_conditional := expr_or_block else_case
 //! simple_pattern_match := pattern "then" simple_conditional
-//! else_clause := "else" expr_or_block
+//! else_case := "else" expr_or_block
 //!
-//! cond_block := "{" (cond_arm comma_or_delimiter)* "}"
+//! cond_block := "{" (cond_arm comma_or_delimiter)* ("else" expr_or_block) "}"
 //! cond_arm := condition guard "then" expr_or_block
 //! condition := "is" pattern | operator precedence_expr | expression
 //! guard := "if" expression
@@ -811,54 +812,44 @@ impl Parser {
   #[recursive_descent_method]
   fn if_expr(&mut self) -> ParseResult<Expression> {
     expect!(self, Keyword(Kw::If));
-    let discriminator = Box::new({
-      self.restrictions.no_struct_literal = true;
-      let x = self.discriminator();
-      self.restrictions.no_struct_literal = false;
-      x?
-    });
-
-    let body = Box::new(match self.token_handler.peek_kind() {
-      Keyword(Kw::Then) => self.conditional()?,
-      Keyword(Kw::Is) => self.simple_pattern_match()? ,
-      _ => self.guard_block()?
-    });
-
+    let discriminator = if let LCurlyBrace = self.token_handler.peek_kind() {
+      Some(Box::new(self.expression()?))
+    } else {
+      None
+    };
+    let body = if_expr_body()?;
     Ok(Expression::new(self.id_store.fresh(), ExpressionKind::IfExpression { discriminator, body }))
   }
 
   #[recursive_descent_method]
-  fn discriminator(&mut self) -> ParseResult<Discriminator> {
-    let lhs = self.prefix_expr()?;
-    let ref next = self.token_handler.peek_kind();
-    Ok(if let Some(op) = BinOp::from_sigil_token(next) {
-      self.token_handler.next();
-      Discriminator::BinOp(lhs, op)
-    } else {
-      Discriminator::Simple(lhs)
-    })
+  fn if_expr_body(&mut self) -> ParseResult<IfExpressionBody> {
+    match self.token_handler.peek_kind() {
+      Keyword(Kw::Then) => self.simple_conditional()?,
+      Keyword(Kw::Is) => self.simple_pattern_match()? ,
+      _ => self.cond_block()?
+    }
   }
 
   #[recursive_descent_method]
-  fn conditional(&mut self) -> ParseResult<IfExpressionBody> {
+  fn simple_conditional(&mut self) -> ParseResult<IfExpressionBody> {
     expect!(self, Keyword(Kw::Then));
-    let then_clause = self.expr_or_block()?;
-    let else_clause = self.else_clause()?;
-    Ok(IfExpressionBody::SimpleConditional(then_clause, else_clause))
+    let then_case = self.expr_or_block()?;
+    let else_case = self.else_case()?;
+    Ok(IfExpressionBody::SimpleConditional {then_case, else_case })
   }
 
   #[recursive_descent_method]
   fn simple_pattern_match(&mut self) -> ParseResult<IfExpressionBody> {
     expect!(self, Keyword(Kw::Is));
-    let pat = self.pattern()?;
+    let pattern = self.pattern()?;
     expect!(self, Keyword(Kw::Then));
-    let then_clause = self.expr_or_block()?;
-    let else_clause = self.else_clause()?;
-    Ok(IfExpressionBody::SimplePatternMatch(pat, then_clause, else_clause))
+    let then_case = self.expr_or_block()?;
+    let else_case = self.else_case()?;
+    Ok(IfExpressionBody::SimplePatternMatch { pattern, then_case, else_case })
   }
 
   #[recursive_descent_method]
-  fn else_clause(&mut self) -> ParseResult<Option<Block>> {
+  fn else_case(&mut self) -> ParseResult<Option<Block>> {
     Ok(if let Keyword(Kw::Else) = self.token_handler.peek_kind() {
       self.token_handler.next();
       Some(self.expr_or_block()?)
@@ -868,7 +859,7 @@ impl Parser {
   }
 
   #[recursive_descent_method]
-  fn guard_block(&mut self) -> ParseResult<IfExpressionBody> {
+  fn cond_block(&mut self) -> ParseResult<IfExpressionBody> {
     //TODO - delimited! isn't sophisticated enough to do thisa
     //let guards = delimited!(self, LCurlyBrace, guard_arm, Comma, RCurlyBrace);
     expect!(self, LCurlyBrace);