[Haskell-beginners] Definition of last: why complicate it?

Daniel Fischer daniel.is.fischer at googlemail.com
Fri Apr 4 19:53:17 UTC 2014

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.


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