Create 2025-03-04-Query_compiler.md

Author: mohanr

_posts/2025-03-04-Query_compiler.md

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+---
+layout: post
+title: Query compilers
+published: false
+---
+
+## OCaml Meta programming
+
+My simplest code generator. There is a scarcity of tutorials that explain OCaml PPX and its variants.
+It is a pity as I believe there exist some good code generation usecases. 
+
+{% highlight ocaml%}
+
+let function_generator ~ctxt _input _is_private =
+  let loc = Expansion_context.Deriver.derived_item_loc ctxt in
+  let expr = [%expr [1; 2; 3]] in
+  let pattern = ppat_var ~loc { txt = "Pattern_test"; loc } in
+  let match_expr =
+    [%expr
+      match [%e expr] with
+      | [%p pattern] -> [%e evar ~loc "Pattern_test"]
+    ]
+  in
+  let code =
+  [[Ast_builder.Default.value_binding ~loc ~pat:pattern ~expr:match_expr] |> pstr_value ~loc Nonrecursive ]
+  in
+  List.iter (fun item ->
+    Format.printf "Generated Code: %a@." Pprintast.structure_item item
+  ) code;
+  code
+
+let generator () = Deriving.Generator.V2.make (args ()) function_generator
+let _ = Deriving.add "name" ~str_type_decl:(generator ())
+
+{% endhighlight %}