From 4b7d8d5d2ae77219793efc240f941b93ce774c01 Mon Sep 17 00:00:00 2001
From: Arnaud Spiwack <arnaud@spiwack.net>
Date: Thu, 18 Nov 2021 12:04:14 +0100
Subject: [PATCH] wip: virtual metrics

---
 prometheus-client/src/Prometheus/Metric.hs    | 18 +++++++++++++++
 .../src/Prometheus/Metric/Gauge.hs            | 23 ++++++++++++++-----
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/prometheus-client/src/Prometheus/Metric.hs b/prometheus-client/src/Prometheus/Metric.hs
index f3cbfbc..e57918c 100644
--- a/prometheus-client/src/Prometheus/Metric.hs
+++ b/prometheus-client/src/Prometheus/Metric.hs
@@ -1,7 +1,9 @@
 {-# language GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE DeriveFunctor #-}
 
 module Prometheus.Metric (
     Metric (..)
+,   metricIO
 ,   Sample (..)
 ,   SampleGroup (..)
 ,   SampleType (..)
@@ -61,6 +63,22 @@ newtype Metric s =
       --    This is the data that will be stored by Prometheus. 
       construct :: IO (s, IO [SampleGroup])
     }
+   deriving (Functor)
+
+-- | Use this Applicative instance to combine metrics, in order to export
+-- a group of metric as a single value.
+instance Applicative Metric where
+  pure x = Metric $ return (x, return [])
+  (Metric iof) <*> (Metric iox) = Metric $ do
+    (f, fsampling) <- iof
+    (x, xsampling) <- iox
+    return (f x, fsampling <> xsampling)
 
 instance NFData a => NFData (Metric a) where
   rnf (Metric a) = a `seq` ()
+
+-- | What should be the name of this function?
+metricIO :: IO (Metric a) -> Metric a
+metricIO iometric = Metric $ do
+  Metric create <- iometric
+  create
diff --git a/prometheus-client/src/Prometheus/Metric/Gauge.hs b/prometheus-client/src/Prometheus/Metric/Gauge.hs
index c0fd80f..dc66de2 100644
--- a/prometheus-client/src/Prometheus/Metric/Gauge.hs
+++ b/prometheus-client/src/Prometheus/Metric/Gauge.hs
@@ -29,9 +29,9 @@ instance NFData Gauge where
 
 -- | Create a new gauge metric with a given name and help string.
 gauge :: Info -> Metric Gauge
-gauge info = Metric $ do
+gauge info = metricIO $ do
     ioref <- IORef.newIORef 0
-    return (MkGauge ioref, collectGauge info ioref)
+    return $ MkGauge ioref <$ virtualGauge info (IORef.readIORef ioref)
 
 withGauge :: MonadMonitor m
           => Gauge
@@ -71,11 +71,22 @@ getGauge (MkGauge ioref) = liftIO $ IORef.readIORef ioref
 setGaugeToDuration :: (MonadIO m, MonadMonitor m) => Gauge -> m a -> m a
 setGaugeToDuration metric io = do
     (result, duration) <- timeAction io
-    setGauge metric duration 
+    setGauge metric duration
     return result
 
-collectGauge :: Info -> IORef.IORef Double -> IO [SampleGroup]
-collectGauge info c = do
-    value <- IORef.readIORef c
+collectGauge :: Info -> IO Double -> IO [SampleGroup]
+collectGauge info measure = do
+    value <- measure
     let sample = Sample (metricName info) [] (BS.fromString $ show value)
     return [SampleGroup info GaugeType [sample]]
+
+data VirtualGauge = VirtualGauge
+
+instance NFData VirtualGauge where
+  rnf VirtualGauge = ()
+
+-- | Create a new gauge metric. Useful to build a gauge backed by an external
+-- store. MORE EXPLAINING TO DO.
+virtualGauge :: Info -> IO Double -> Metric VirtualGauge
+virtualGauge info measure = Metric $ do
+    return (VirtualGauge, collectGauge info measure)