Introduce parser::Fail

src/parse/mod.rs

@@ -1,12 +1,17 @@
 pub mod ast;
+pub mod number_literal;
 pub mod parser;
 pub mod problems;
 pub mod string_literal;
 
 use parse::ast::Expr;
+// use parse::number_literal::number_literal;
 use parse::parser::Parser;
 use parse::string_literal::string_literal;
 
 pub fn expr<'a>() -> impl Parser<'a, Expr<'a>> {
+    // parser::one_of2(string_literal(), number_literal())
     string_literal()
 }
+
+const KW_IF: &'static str = "if";