parser: initial expression parsing logic

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

src/parser/statement.rs

@@ -0,0 +1,87 @@
+use super::Parser;
+use crate::ast::*;
+use crate::lexer::{
+    TokenKeyword,
+    TokenKind,
+    TokenSymbol,
+};
+
+impl<'a> Parser<'a> {
+    /// statement ::= static | let | expr
+    pub(super) fn parse_statement(&mut self) -> Option<Statement> {
+        use TokenKeyword::*;
+
+        Some(match self.peek_token().kind {
+            TokenKind::Keyword(Static) => Statement::Static(self.parse_static()?),
+            TokenKind::Keyword(Let) => Statement::Let(self.parse_let()?),
+            _ => Statement::Expr(self.parse_expr_ln()?),
+        })
+    }
+
+    /// static ::="static" let
+    pub(super) fn parse_static(&mut self) -> Option<Let> {
+        self.next_token();
+
+        if self.peek_token().kind != TokenKind::Keyword(TokenKeyword::Let) {
+            self.error_expected_peek("let");
+            return None;
+        }
+
+        self.parse_let()
+    }
+
+    /// let ::= "let" identWithTy "=" expr
+    pub(super) fn parse_let(&mut self) -> Option<Let> {
+        self.next_token();
+
+        let (name, ty) = self.parse_ident_with_ty()?;
+
+        let expr = if self.skip_token(TokenKind::Symbol(TokenSymbol::Eq)) {
+            self.parse_expr_ln()
+        } else if self.skip_token(TokenKind::Newline) {
+            None
+        } else {
+            self.error_expected_peek("= or newline");
+            return None;
+        };
+
+        Some(Let { name, ty, expr })
+    }
+}
+
+#[test]
+fn test_parse_let() {
+    use Literal::*;
+
+    let mut parser = Parser::new(
+        r#"static let test01: int = 4
+           let test02: char = '6'
+           static let test03: float
+           let test04 = 9"#,
+    );
+    assert_eq!(
+        parser.parse_static(),
+        Some(Let {
+            name: "test01".into(),
+            ty: Ty::Int,
+            expr: Some(Expr::Literal(Int(4)))
+        })
+    );
+    assert_eq!(
+        parser.parse_let(),
+        Some(Let {
+            name: "test02".into(),
+            ty: Ty::Char,
+            expr: Some(Expr::Literal(Char('6')))
+        })
+    );
+    assert_eq!(
+        parser.parse_static(),
+        Some(Let {
+            name: "test03".into(),
+            ty: Ty::Float,
+            expr: None
+        })
+    );
+    assert_eq!(parser.parse_let(), None);
+}