From e596f65e10c76f85a9b0069a79d07015e9184a9f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C5=81ukasz=20Niemier?= <~@hauleth.dev>
Date: Mon, 13 Apr 2026 15:23:35 +0200
Subject: [PATCH] Implement `execute_lazy/{2,3}`

This allow to compute measurements and metadata only if there is anyone
listening for given events. That provides a way to expose metrics that
can be slightly more expensive to compute.

Closes #150
---
 src/telemetry.erl | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/src/telemetry.erl b/src/telemetry.erl
index 8b2484a..be739b1 100644
--- a/src/telemetry.erl
+++ b/src/telemetry.erl
@@ -5,6 +5,8 @@
          persist/0,
          detach/1,
          list_handlers/1,
+         execute_lazy/2,
+         execute_lazy/3,
          execute/2,
          execute/3,
          span/3]).
@@ -165,6 +167,22 @@ If the handler with given ID doesn't exist, `{error, not_found}` is returned.
 detach(HandlerId) ->
     telemetry_handler_table:delete(HandlerId).
 
+execute_lazy(EventName, Func) -> execute_lazy(EventName, ignored, Func).
+
+execute_lazy([_ | _] = EventName, Fallback, Func) when is_function(Func, 0) ->
+    case telemetry_handler_table:list_for_event(EventName) of
+        [] -> Fallback;
+        Handlers ->
+            {Return, Data} = Func(),
+            case Data of
+                ignore ->
+                    [];
+                {Measurements, Metadata} ->
+                    do_execute(Handlers, EventName, Measurements, Metadata)
+            end,
+            Return
+    end.
+
 ?DOC("""
 Emits the event, invoking handlers attached to it.