Make sure GHC.List.last is memory-efficient

by eta-expanding its definition so that GHC optmizes the foldl here.
Also make sure that other uses of last go via foldl as well, to allow
list fusion (tested in T9339). Fixes #10260.

(cherry picked from commit 524ddbdad5816f77b7b719cac0671eebd3473616)

1 files changed, 9 insertions, 2 deletions

diff --git a/libraries/base/GHC/List.hs b/libraries/base/GHC/List.hs
index a712f9e4be..fcc89d3f8f 100644
--- a/libraries/base/GHC/List.hs
+++ b/libraries/base/GHC/List.hs
@@ -84,8 +84,15 @@ last [x]                =  x
 last (_:xs)             =  last xs
 last []                 =  errorEmptyList "last"
 #else
--- use foldl to allow fusion
-last = foldl (\_ x -> x) (errorEmptyList "last")
+-- Use foldl to make last a good consumer.
+-- This will compile to good code for the actual GHC.List.last.
+-- (At least as long it is eta-expaned, otherwise it does not, #10260.)
+last xs = foldl (\_ x -> x) lastError xs
+{-# INLINE last #-}
+-- The inline pragma is required to make GHC remember the implementation via
+-- foldl.
+lastError :: a
+lastError = errorEmptyList "last"
 #endif
 
 -- | Return all the elements of a list except the last one.