Let the list instance fuse

[Bodigrim]

Fixes #76, CC @treeowl.

To measure performance of someFunc Haskell benchmarks often employ code like this:

map someFunc [1..1000] `deepseq` ()

Here we want to measure time of 1000 applications of someFunc, and in order to do so we deepseq the entire list map someFunc [1..1000]. However, in this scenario we are not really interested in materializing the list: if someFunc is relatively fast, allocations are likely to skew measurements. See Bodigrim/tasty-bench#48 (comment) for discussion and various hacky workarounds.

We can do better by making instance NFData [a] able to fuse by rewriting it via foldr. This has a nice side effect of avoiding manual recursion and having a more concise definition as well.

Before the patch

foo :: Int -> ()
foo n = [1..n] `deepseq` ()

compiles to

Rec {
$wgo3 :: [Int] -> (# #)
$wgo3
  = \ (ds_s8hJ :: [Int]) ->
      case ds_s8hJ of {
        [] -> (##);
        : y_i8gZ ys_i8h0 -> case y_i8gZ of { I# ipv_i8gR -> $wgo3 ys_i8h0 }
      }
end Rec }

$wfoo :: Int# -> (# #)
$wfoo
  = \ (ww_s8hT :: Int#) ->
      case ># 1# ww_s8hT of {
        __DEFAULT ->
          letrec {
            go3_a8hF :: Int# -> [Int]
            go3_a8hF
              = \ (x_a8hG :: Int#) ->
                  : (I# x_a8hG)
                    (case ==# x_a8hG ww_s8hT of {
                       __DEFAULT -> go3_a8hF (+# x_a8hG 1#);
                       1# -> []
                     }); } in
          $wgo3 (go3_a8hF 1#);
        1# -> (##)
      }

foo :: Int -> ()
foo
  = \ (n_s8hR :: Int) ->
      case n_s8hR of { I# ww_s8hT ->
      case $wfoo ww_s8hT of { (# #) -> () }
      }

Here one can observe $wgo3 which is forcing a list (essentially this is rnf from instance NFData [a]) and go3_a8hF which produces it. Such code allocates boxed Int and (:) constructors.

After the patch:

foo :: Int -> ()
foo
  = \ (n_a8g8 :: Int) ->
      case n_a8g8 of { I# y_a8ha ->
      case ># 1# y_a8ha of {
        __DEFAULT ->
          joinrec {
            go3_a5N1 :: Int# -> ()
            go3_a5N1 (x_a5N2 :: Int#)
              = case ==# x_a5N2 y_a8ha of {
                  __DEFAULT -> jump go3_a5N1 (+# x_a5N2 1#);
                  1# -> ()
                }; } in
          jump go3_a5N1 1#;
        1# -> ()
      }
      }

Here we do force evaluation of all elements of the list, but do not allocate them.