On Friday 04 April 2014, 20:12:50, Rudy Matela wrote:
According to a stackoverflow answer [1], this should be done automatically by GHC. Why it's still defined like that I don't know: maybe because the code is for when using other compilers, or maybe I misinterpreted the stackoverflow post and GHC is not able to do that.
One can check that, just copy the source (hiding Prelude.last or renaming it) and compile, with -ddump-simpl to get the core GHC produces (it takes a little practice to read core, but it's sufficiently similar to Haskell to start understanding much of it quickly). With -O2, all GHC versions I tried (6.12.3, 7.0.2, 7.2.2, 7.4.2, 7.6.1, 7.6.3) produced (almost¹) the more efficient version from the USE_REPORT_PRELUDE source, but with only -O, none did. If the libraries are guaranteed to be compiled with -O2 when building the compiler, there would be no need for the other source. But since that is not guaranteed, it's better to manually write the better version. Footnote (¹): what GHC produces with -O2 is slightly different, it tests for a singleton list once in the wrapper before the worker is called, but it also creates a rule that when it is called on a list that is known at compile-time to be nonempty, the worker shall be called directly. Cheers, Daniel