feat(experimental:template-language): start an own template language

## CAUTION! This is an experiment

The whole reason for this experiment is to get closer to the http plugin template syntax as linked in #5

There the vscode plugin [has this concept of system variables](https://marketplace.visualstudio.com/items?itemName=humao.rest-client#system-variables) that follow the syntax of a mix of expression and function e.g.

- {{$randomInt min max}}: Returns a random integer between min (included) and max (excluded)
- {{$dotenv [%]variableName}}: Returns the environment value stored in the .env file which exists in the same directory of your .http file.

Both examples show that `$<ident>` is similar to a function name and then a variable argument list is passed without any braces like `()`.

This experiment focuses on the ability to register functions for this very system variable syntax at compile time in an extensible fashion.

done so far:
- lexing and parsing of very basic templates with an expression `{{ var }}`
- basic runtime to interpret the AST
- some tests added
- design of a Visitor pattern for the AST

yet open
- [ ] tests for unhappy path are to less
- [ ] runtime is very incomplete yet, `SysVar` hooked functions are missing e.g. `hello {{ $processEnv HOME }}`

Signed-off-by: Sven Kanoldt <sven@d34dl0ck.me>

Author: sassman

src/curlz/language/ast.rs

@@ -68,3 +68,24 @@ pub enum Stmt<'a> {
     EmitRaw(Spanned<EmitRaw<'a>>),
     EmitExpr(Spanned<EmitExpr<'a>>),
 }
+
+#[cfg(test)]
+pub trait IntoSpanned {
+    fn spanned(self) -> Spanned<Self>
+    where
+        Self: Sized,
+    {
+        Spanned::new(
+            self,
+            Span {
+                start_line: 1,
+                start_col: 0,
+                end_line: 1,
+                end_col: 1,
+            },
+        )
+    }
+}
+
+#[cfg(test)]
+impl<T> IntoSpanned for T {}

src/curlz/language/ast_visitor.rs

@@ -0,0 +1,33 @@
+/*!
+this module contains AST related tooling such as the visitor trait and
+the double dispatch (impl of [`AstVisitAcceptor`]) for all AST nodes.
+*/
+use crate::language::ast;
+
+pub trait AstVisitAcceptor<'ast> {
+    fn accept<V: AstVisit<'ast>>(&self, visitor: &mut V);
+}
+
+pub trait AstVisit<'ast> {
+    fn visit_stmt(&mut self, _stmt: &ast::Stmt<'ast>) {}
+    fn visit_expr(&mut self, _expr: &ast::Expr<'ast>) {}
+    fn visit_emit_raw(&mut self, _raw: &ast::EmitRaw<'ast>) {}
+}
+
+impl<'ast> AstVisitAcceptor<'ast> for ast::Stmt<'ast> {
+    fn accept<V: AstVisit<'ast>>(&self, visitor: &mut V) {
+        visitor.visit_stmt(self);
+    }
+}
+
+impl<'ast> AstVisitAcceptor<'ast> for ast::Expr<'ast> {
+    fn accept<V: AstVisit<'ast>>(&self, visitor: &mut V) {
+        visitor.visit_expr(self);
+    }
+}
+
+impl<'ast> AstVisitAcceptor<'ast> for ast::EmitRaw<'ast> {
+    fn accept<V: AstVisit<'ast>>(&self, visitor: &mut V) {
+        visitor.visit_emit_raw(self);
+    }
+}

src/curlz/language/mod.rs

@@ -1,4 +1,5 @@
 mod ast;
+mod ast_visitor;
 mod lexer;
 mod parser;
 mod runtime;

src/curlz/language/runtime.rs

@@ -1,97 +1,113 @@
 use crate::language::ast;
-use crate::language::ast::Spanned;
+use crate::language::ast_visitor::{AstVisit, AstVisitAcceptor};
+use std::borrow::Cow;
 use std::collections::HashMap;
 
 #[derive(Default)]
 struct Runtime<'source> {
     // todo: maybe an `Ident` should be the key for variables
     vars: HashMap<&'source str, ast::Value>,
-    interpreted: Vec<u8>,
+    output: Vec<u8>,
 }
 
-trait Visitable<'source> {
-    fn accept<V: Visitor<'source>>(&self, visitor: &mut V);
-}
-
-trait Visitor<'source> {
-    fn visit_stmt(&mut self, stmt: &ast::Stmt<'source>);
-    fn visit_expr(&mut self, expr: &ast::Expr<'source>);
-    fn visit_emit_raw(&mut self, raw: &ast::EmitRaw<'source>);
-}
+impl<'source> Runtime<'source> {
+    /// registers a variable with a given `id` that is the variable identifier
+    pub fn with_variable(mut self, id: &'source str, var: impl Into<ast::Value>) -> Self {
+        self.vars.insert(id, var.into());
 
-impl<'source> Visitable<'source> for ast::Stmt<'source> {
-    fn accept<V: Visitor<'source>>(&self, visitor: &mut V) {
-        visitor.visit_stmt(self);
+        self
     }
-}
 
-impl<'source> Visitable<'source> for ast::Expr<'source> {
-    fn accept<V: Visitor<'source>>(&self, visitor: &mut V) {
-        visitor.visit_expr(self);
+    /// returns the rendered template as a string in form of a `Cow<'_, str>`
+    pub fn rendered(&mut self) -> Cow<'_, str> {
+        String::from_utf8_lossy(self.output.as_slice())
     }
-}
 
-impl<'source> Visitable<'source> for ast::EmitRaw<'source> {
-    fn accept<V: Visitor<'source>>(&self, visitor: &mut V) {
-        visitor.visit_emit_raw(self);
+    #[cfg(test)]
+    pub fn render(&mut self, source: &'source str) -> Cow<'_, str> {
+        use crate::language::parser::Parser;
+
+        let parsed = Parser::new(source).parse().unwrap();
+        parsed.accept(self);
+
+        self.rendered()
     }
 }
 
-impl<'source> Visitor<'source> for Runtime<'source> {
+impl<'source> AstVisit<'source> for Runtime<'source> {
     fn visit_stmt(&mut self, stmt: &ast::Stmt<'source>) {
+        use ast::Stmt::*;
+
         match stmt {
-            ast::Stmt::Template(spanned) => {
+            Template(spanned) => {
                 for s in spanned.node.children.as_slice() {
                     s.accept(self);
                 }
             }
-            ast::Stmt::EmitRaw(spanned) => spanned.node.accept(self),
-            ast::Stmt::EmitExpr(spanned) => spanned.node.expr.accept(self),
+            EmitRaw(spanned) => spanned.node.accept(self),
+            EmitExpr(spanned) => spanned.node.expr.accept(self),
         }
     }
 
     fn visit_expr(&mut self, expr: &ast::Expr<'source>) {
-        use ast::Expr;
+        use ast::Expr::*;
 
         match expr {
-            Expr::SysVar(var) => todo!(),
-            Expr::Var(var) => {
+            SysVar(var) => todo!(),
+            Var(var) => {
                 if let Some(var) = self.vars.get(var.node.id) {
-                    self.interpreted
+                    self.output
                         .extend_from_slice(var.as_str().unwrap().as_bytes());
                 }
             }
-            Expr::Const(_) => {
+            Const(_) => {
                 todo!()
             }
-            Expr::Call(_) => {
+            Call(_) => {
                 todo!()
             }
         }
     }
 
     fn visit_emit_raw(&mut self, raw: &ast::EmitRaw<'source>) {
-        self.interpreted.extend_from_slice(raw.raw.as_bytes());
+        self.output.extend_from_slice(raw.raw.as_bytes());
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::language::parser::Parser;
+    use crate::language::ast::IntoSpanned;
+
+    #[test]
+    fn test_expr_var() {
+        let mut runtime = Runtime::default().with_variable("foo", "John");
+        let expr = ast::Expr::Var(ast::Var { id: "foo" }.spanned());
+
+        expr.accept(&mut runtime);
+        assert_eq!(runtime.rendered(), "John");
+    }
+
+    #[test]
+    fn test_expr_sys_var() {
+        assert_eq!(
+            Runtime::default().render("{{ $processEnv HOME }}"),
+            env!("HOME")
+        );
+    }
+
+    #[test]
+    fn test_whole_template() {
+        assert_eq!(
+            Runtime::default()
+                .with_variable("world", "John")
+                .render("hello {{ world }}"),
+            "hello John"
+        );
+    }
 
     #[test]
-    fn test_x() {
-        let mut p = Parser::new("hello {{ world }}");
-        let stmt = p.parse().unwrap();
-        let mut runtime = Runtime::default();
-        runtime
-            .vars
-            .insert("world", ast::Value::String("John".to_string()));
-
-        stmt.accept(&mut runtime);
-
-        let result = String::from_utf8_lossy(runtime.interpreted.as_slice());
-        assert_eq!(result, "hello John");
+    fn test_whole_template_unhappy() {
+        assert_eq!(Runtime::default().render("hello {{ world }}"), "hello ");
     }
 }