feat: add ast sample

Makefile

@@ -36,3 +36,7 @@ bench:
 
 .PHONY: default install uninstall reinstall clean test doc bench
 .PHONY: all-supported-ocaml-versions opam-release
+
+.PHONY: $(TARGET)
+example-%: 
+	DUNE_CONFIG__GLOBAL_LOCK=disabled opam exec -- dune exec $*-example

doc/dune

@@ -1,2 +1,12 @@
 (documentation
  (package ppxlib))
+
+(rule
+ (alias doc)
+ (deps
+   (glob_files ./images/*))
+ (action
+   (progn
+     (system "echo 'hello'")
+     (system "mkdir -p %{project_root}/_doc/_html/ppxlib/assets/images")
+     (system "cp -R ./images/ %{project_root}/_doc/_html/ppxlib/assets/images/"))))

doc/example-ast-building.mld

@@ -0,0 +1,173 @@
+{0 Building AST}
+
+{1 Table of Contents}
+
+- {{!section-description} Description}
+- {{!section-"building-asts-with-pure-ocaml"} Building ASTs with Pure OCaml}
+{ul {- {{!section-"example-building-a-simple-integer-ast-manually"} Example: Building a Simple Integer AST Manually}}}
+
+- {{!section-"building-asts-with-ast_builder"} Building ASTs with `AST_builder`}
+{ul {- {{!section-"example-1-using-pexp_constant-for-integer-ast"} Example 1: Using `pexp_constant` for Integer AST}}}
+{ul {- {{!section-"example-2-using-eint-for-simplified-integer-ast"} Example 2: Using `eint` for Simplified Integer AST}}}
+
+- {{!section-"using-metaquot-for-ast-construction"} Using Metaquot for AST Construction}
+{ul {- {{!section-"example-building-an-integer-ast-with-metaquot"} Example: Building an Integer AST with Metaquot}}}
+
+- {{!section-"using-anti-quotations-in-metaquot"} Using Anti-Quotations in Metaquot}
+{ul {- {{!section-"example-inserting-dynamic-expressions-with-anti-quotations"} Example: Inserting Dynamic Expressions with Anti-Quotations}}}
+
+- {{!section-"building-complex-expressions"} Building Complex Expressions}
+{ul {- {{!section-"example-1-constructing-a-let-expression-with-ast_builder"} Example 1: Constructing a Let Expression with `AST_builder`}}}
+{ul {- {{!section-"example-2-constructing-a-let-expression-with-metaquot"} Example 2: Constructing a Let Expression with Metaquot}}}
+
+- {{!section-conclusion} Conclusion}
+
+{1:description Description}
+
+Building an AST (Abstract Syntax Tree) is a fundamental part of creating a PPX in OCaml. You'll need to construct an AST to represent the code you want to generate or transform.
+
+For example, if you want to generate the following code:
+
+{[
+let zero = [%int 0]
+]}
+
+and replace the extension point `[%int 0]` with `0` to produce `let zero = 0`, you’ll need to build an AST that represents this transformation.
+
+There are several methods to build an AST. We’ll discuss three approaches:
+
+- {b Building ASTs with Pure OCaml}
+- {b Building ASTs with `AST_builder`}
+- {b Using Metaquot for AST Construction}
+
+{1:building-asts-with-pure-ocaml Building ASTs with Low-Level Builders}
+
+The most fundamental way to build an AST is to manually construct it using Low-Level Builders data structures.
+
+{2:example-building-a-simple-integer-ast-manually Example: Building a Simple Integer AST Manually}
+
+{{:https://github.com/ocaml-ppx/ppxlib/tree/main/examples/1-AST/a%20-%20Building%20AST/building_ast.ml#L5-L16} 🔗 Sample Code}
+
+{[
+let zero ~loc : Ppxlib_ast.Ast.expression =
+  {
+    pexp_desc = Pexp_constant (Pconst_integer ("0", None));
+    pexp_loc = loc;
+    pexp_loc_stack = [];
+    pexp_attributes = [];
+  }
+]}
+
+While this method provides full control over the AST, it is verbose and less maintainable.
+
+{1:building-asts-with-ast_builder Building ASTs with `AST_builder`}
+
+PPXLib provides the `AST_builder` module, which simplifies the process of building ASTs by providing helper functions.
+
+{2:example-1-using-pexp_constant-for-integer-ast Example 1: Using `pexp_constant` for Integer AST}
+
+{{:https://github.com/ocaml-ppx/ppxlib/tree/main/examples/1-AST/a%20-%20Building%20AST/building_ast.ml#L18-L24} 🔗 Sample Code}
+
+{[
+let one ~loc =
+  Ast_builder.Default.pexp_constant ~loc (Parsetree.Pconst_integer ("1", None))
+]}
+
+This method is more readable and concise compared to the pure OCaml approach.
+
+{2:example-2-using-eint-for-simplified-integer-ast Example 2: Using `eint` for Simplified Integer AST}
+
+{{:https://github.com/ocaml-ppx/ppxlib/tree/main/examples/1-AST/a%20-%20Building%20AST/building_ast.ml#L26-L31} 🔗 Sample Code}
+
+{[
+let two ~loc = Ast_builder.Default.eint ~loc 2
+]}
+
+{b Tip:} `eint` is an abbreviation for expression (`e`) integer (`int`).
+
+{1:using-metaquot-for-ast-construction Using Metaquot for AST Construction}
+
+Metaquot is a syntax extension that allows you to write ASTs in a more natural and readable way.
+
+{2:example-building-an-integer-ast-with-metaquot Example: Building an Integer AST with Metaquot}
+
+{{:https://github.com/ocaml-ppx/ppxlib/tree/main/examples/1-AST/a%20-%20Building%20AST/building_ast.ml#L33-L38} 🔗 Sample Code}
+
+{[
+let three ~loc = [%expr 3]
+]}
+
+{b Tip:} Metaquot is highly readable and intuitive but is static. For dynamic values, use Anti-Quotations.
+
+{2:using-anti-quotations-in-metaquot Using Anti-Quotations in Metaquot}
+
+Anti-Quotations allow you to insert dynamic expressions into your Metaquot ASTs.
+
+{3:example-inserting-dynamic-expressions-with-anti-quotations Example: Inserting Dynamic Expressions with Anti-Quotations}
+
+{{:https://github.com/ocaml-ppx/ppxlib/tree/main/examples/1-AST/a%20-%20Building%20AST/building_ast.ml#L72-L77} 🔗 Sample Code}
+
+{[
+let anti_quotation_expr expr = [%expr 1 + [%e expr]]
+]}
+
+For example, to insert the AST for `1`:
+
+{[
+let _ =
+  print_endline
+    ("\nLet expression with metaquot and anti-quotation: "
+    ^ Astlib.Pprintast.string_of_expression (anti_quotation_expr (one ~loc)))
+]}
+
+{1:building-complex-expressions Building Complex Expressions}
+
+Beyond simple expressions, you may need to build more complex ASTs, such as `let` expressions.
+
+{2:example-1-constructing-a-let-expression-with-ast_builder Example 1: Constructing a Let Expression with `AST_builder`}
+
+{{:https://github.com/ocaml-ppx/ppxlib/tree/main/examples/1-AST/a%20-%20Building%20AST/building_ast.ml#L40-L60} 🔗 Sample Code}
+
+{[
+let let_expression =
+  let expression =
+    Ast_builder.Default.pexp_constant ~loc:Location.none
+      (Pconst_integer ("3", None))
+  in
+  let pattern =
+    Ast_builder.Default.ppat_var ~loc:Location.none
+      (Ast_builder.Default.Located.mk ~loc:Location.none "foo")
+  in
+  let let_binding =
+    Ast_builder.Default.value_binding ~loc:Location.none ~pat:pattern
+      ~expr:expression
+  in
+  Ast_builder.Default.pexp_let ~loc:Location.none Nonrecursive [ let_binding ]
+    (Ast_builder.Default.eunit ~loc:Location.none)
+]}
+
+{2:example-2-constructing-a-let-expression-with-metaquot Example 2: Constructing a Let Expression with Metaquot}
+
+{{:https://github.com/ocaml-ppx/ppxlib/tree/main/examples/1-AST/a%20-%20Building%20AST/building_ast.ml#L62-L70} 🔗 Sample Code}
+
+{[
+let let_expression =
+  [%expr
+    let foo = 3 in
+    ()]
+]}
+
+This approach is shorter and easier to understand.
+
+{1:conclusion Conclusion}
+
+In this section, we explored three methods for building ASTs:
+
+- {b Pure OCaml}: The most basic but verbose approach.
+- {b Using `AST_builder`}: A more readable and maintainable option.
+- {b Using Metaquot}: The most intuitive method, especially when combined with Anti-Quotations for dynamic values.
+
+Each method has its strengths, so choose the one that best fits your needs. Understanding all three will give you greater flexibility in creating effective and maintainable PPXs.
+
+{2 Next Steps}
+On the next section, we will learn how to destructure an AST. {{:../b%20-%20Destructing%20AST/README.md} Read more}

doc/example-ast-destructing.mld

@@ -0,0 +1,171 @@
+{0 Destructuring AST}
+
+{1:table-of-contents Table of Contents}
+
+- {{!section-description} Description}
+- {{!section-"ast-structure-pattern-matching"} AST Structure Pattern Matching}
+{ul {- {{!section-"example-matching-integer-payload-manually"} Example: Matching Integer Payload Manually}}}
+
+- {{!section-"using-ast_pattern-high-level-destructors"} Using `Ast_pattern` High-Level Destructors}
+{ul {- {{!section-"example-1-matching-integer-payload-with-ast_pattern"} Example 1: Matching Integer Payload with `Ast_pattern`}}}
+{ul {- {{!section-"example-2-simplifying-matching-with-eint"} Example 2: Simplifying Matching with `eint`}}}
+
+- {{!section-"using-metaquot"} Using Metaquot}
+{ul {- {{!section-"example-1-matching-integer-payload-with-metaquot"} Example 1: Matching Integer Payload with Metaquot}}}
+{ul {- {{!section-"example-2-matching-complex-expressions-with-metaquot-and-anti-quotations"} Example 2: Matching Complex Expressions with Metaquot and Anti-Quotations}}}
+
+- {{!section-"conclusion"} Conclusion}
+
+{1:description Description}
+
+Destructuring an AST (Abstract Syntax Tree) is essential when creating a PPX (preprocessor extension) in OCaml. To generate or transform code, you must first break down the AST to understand and manipulate its structure.
+
+For example, if you want to transform this code:
+
+{[
+let one = [%one]
+]}
+
+into:
+
+{[
+let one = 1
+]}
+
+You’ll need to destructure the AST representing the extension point (`[%one]`) to replace it with `1`.
+There are several ways to destructure an AST. We’ll explore three methods:
+
+- {b AST Structure Pattern Matching}
+- {b Using `Ast_pattern` High-Level Destructors}
+- {b Using Metaquot}
+
+{1:ast-structure-pattern-matching AST Structure Pattern Matching}
+
+The most fundamental method for destructuring an AST in PPXLib is by directly matching on the AST’s structure.
+
+{2:example-matching-integer-payload-manually Example: Matching Integer Payload Manually}
+
+{[:link: Sample Code](./destructuring_ast.ml#L11-L26)}
+
+Let’s say we want to destructure an AST representing the integer `1`:
+
+{[
+let match_int_payload ~loc payload =
+  match payload with
+  | PStr
+      [
+        {
+          pstr_desc =
+            Pstr_eval
+              ({ pexp_desc = Pexp_constant (Pconst_integer (value, None)); _ }, _);
+          _;
+        };
+      ] -> (
+      try Ok (value |> int_of_string)
+      with Failure _ ->
+        Error (Location.Error.createf ~loc "Value is not a valid integer"))
+  | _ -> Error (Location.Error.createf ~loc "Wrong pattern")
+]}
+
+1. {b Pattern Matching the Payload}:
+    - Begins by matching the `payload` with the expected structure.
+    - The pattern expects a structure (`PStr`) containing a single item.
+2. {b Destructuring the Structure Item}:
+    - Matches the `pstr_desc` field, expecting an evaluated expression (`Pstr_eval`).
+    - The expression should be a constant integer (`Pexp_constant` with `Pconst_integer`).
+    - Captures the integer value as a string in `value`.
+3. {b Handling the Matched Value}:
+    - Converts the `value` to an integer and returns `Ok` if successful.
+    - If conversion fails, returns an error message.
+4. {b Handling Mismatched Patterns}:
+    - If the `payload` doesn’t match the expected structure, it returns an error.
+
+While this method is powerful, it can be verbose and difficult to maintain as patterns become more complex.
+
+{1:using-ast_pattern-high-level-destructors Using `Ast_pattern` High-Level Destructors}
+
+To make AST destructuring more readable, PPXLib provides the `Ast_pattern` module, which offers high-level destructors.
+
+{2:example-1-matching-integer-payload-with-ast_pattern Example 1: Matching Integer Payload with `Ast_pattern`}
+
+{[:link: Sample Code](./destructuring_ast.ml#L37-L40)}
+
+Let’s destructure the same integer `1` AST using `Ast_pattern`:
+
+{[
+open Ppxlib
+
+let match_int_payload =
+  let open Ast_pattern in
+  pstr (pstr_eval (pexp_constant (pconst_integer (string "1") none)) nil ^:: nil)
+]}
+
+This code achieves the same result as the previous example but in a more concise and readable way.
+
+- {b `PStr`} becomes `pstr`
+- {b `Pstr_eval`} becomes `pstr_eval`
+- {b `Pexp_constant`} becomes `pexp_constant`
+- {b `Pconst_integer`} becomes `pconst_integer`
+
+{2:example-2-simplifying-matching-with-eint Example 2: Simplifying Matching with `eint`}
+
+{[:link: Sample Code](./destructuring_ast.ml#L40-L49)}
+
+You can further simplify it:
+
+{[
+let match_int_payload =
+  let open Ast_pattern in
+  pstr (pstr_eval (eint (int 1)) nil ^:: nil)
+]}
+
+Using `eint` instead of `pexp_constant` and `pconst_integer` provides better type safety. The `int` wildcard captures the integer value.
+
+{1:using-metaquot Using Metaquot}
+
+Metaquot is a syntax extension that allows you to write and destructure ASTs more intuitively.
+
+{2:example-1-matching-integer-payload-with-metaquot Example 1: Matching Integer Payload with Metaquot}
+
+{[:link: Sample Code](./destructuring_ast.ml#L51-L60)}
+
+Let’s destructure the same integer `1` AST with Metaquot:
+
+{[
+let match_int_payload expr =
+  match expr with
+  | [%expr 1] -> Ok 1
+  | _ -> Error (Location.Error.createf ~loc:expr.pexp_loc "Wrong pattern")
+]}
+
+{2:example-2-matching-complex-expressions-with-metaquot-and-anti-quotations Example 2: Matching Complex Expressions with Metaquot and Anti-Quotations}
+
+{[:link: Sample Code](./destructuring_ast.ml#L79-L90)}
+
+For example, to match any expression of the form `1 + <int>`:
+
+{[
+let match_int_payload expr =
+  match expr with
+  | [%expr 1 + [%e? e]] -> (
+      match e with
+      | { pexp_desc = Pexp_constant (Pconst_integer (value, None)); _ } ->
+          Ok (1 + int_of_string value)
+      | _ -> Error (Location.Error.createf ~loc:e.pexp_loc "Invalid integer"))
+  | _ -> Error (Location.Error.createf ~loc:expr.pexp_loc "Wrong pattern")
+]}
+
+Metaquot simplifies the process, making the AST patterns more readable, especially for complex structures.
+
+{1:conclusion Conclusion}
+
+In this section, we explored different methods to destructure an AST using PPXLib:
+
+- {b AST Structure Pattern Matching}: Powerful but verbose.
+- {b Using `Ast_pattern` High-Level Destructors}: More readable and maintainable.
+- {b Using Metaquot}: Intuitive and effective for both simple and complex patterns.
+
+There’s no right way to destructure an AST, choose the approach that best fits your use case. Understanding all these methods is valuable for creating robust and maintainable PPXs.
+
+{1:next-steps Next Steps}
+On the next section, we will learn how to write a PPX. {{:../../2%20-%20Writing%20PPXs/README.md} Read more}