From ac9937db2229be16c6f689b0282a563d5b45b539 Mon Sep 17 00:00:00 2001
From: Milan Bouchet-Valat <nalimilan@club.fr>
Date: Sat, 9 Sep 2023 15:10:08 +0200
Subject: [PATCH] Call user function only once in `mean`

Use an internal type to initiate promotion instead of using the type
returned by user-provided function applied to the first element.
This avoid calling the function twice, which can be confusing.
---
 src/Statistics.jl | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/Statistics.jl b/src/Statistics.jl
index 560b227d..30c154ce 100644
--- a/src/Statistics.jl
+++ b/src/Statistics.jl
@@ -178,7 +178,10 @@ if !isdefined(Base, :mean)
     """
     mean(A::AbstractArray; dims=:) = _mean(identity, A, dims)
 
+    struct _InitType end
+
     _mean_promote(x::T, y::S) where {T,S} = convert(promote_type(T, S), y)
+    _mean_promote(x::_InitType, y::Any) = y/1
 
     # ::Dims is there to force specializing on Colon (as it is a Function)
     function _mean(f, A::AbstractArray, dims::Dims=:) where Dims
@@ -188,8 +191,7 @@ if !isdefined(Base, :mean)
         else
             n = mapreduce(i -> size(A, i), *, unique(dims); init=1)
         end
-        x1 = f(first(A)) / 1
-        result = sum(x -> _mean_promote(x1, f(x)), A, dims=dims)
+        result = sum(x -> _mean_promote(_InitType(), f(x)), A, dims=dims)
         if dims === (:)
             return result / n
         else