Two ways to filter a list in Haskell

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<p>What do two different ways of filtering a list compile to?</p>

<p>(Example: take a list of tuples (a,b), and return a list of a for every (a,b) where b > 1.)</p>

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`map fst . filter ((>1) . snd)`
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`[a | (a,b) <- x, b > 1]`
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<pre>
Rec {
Main.f1_go [Occ=LoopBreaker] :: [(Int, Int)] -> [Int]
[GblId, Arity=1, Caf=NoCafRefs, Str=DmdType <S,1*U>]
Main.f1_go =
  \ (ds_a164 :: [(Int, Int)]) ->
    case ds_a164 of _ [Occ=Dead] {
      [] -> GHC.Types.[] @ Int;
      : y_a169 ys_a16a ->
        case y_a169 of _ [Occ=Dead] { (ds1_a16v, y1_a16w) ->
        case y1_a16w of _ [Occ=Dead] { GHC.Types.I# x_a173 ->
        case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.># x_a173 1)
        of _ [Occ=Dead] {
          False -> Main.f1_go ys_a16a;
          True -> GHC.Types.: @ Int ds1_a16v (Main.f1_go ys_a16a)
        }
        }
        }
    }
end Rec }


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<pre>
Rec {
Main.f2_go [Occ=LoopBreaker] :: [(Int, Int)] -> [Int]
[GblId, Arity=1, Caf=NoCafRefs, Str=DmdType <S,1*U>]
Main.f2_go =
  \ (ds_a164 :: [(Int, Int)]) ->
    case ds_a164 of _ [Occ=Dead] {
      [] -> GHC.Types.[] @ Int;
      : y_a169 ys_a16a ->
        case y_a169 of _ [Occ=Dead] { (a_aEq, b_aEr) ->
        case b_aEr of _ [Occ=Dead] { GHC.Types.I# x_a173 ->
        case GHC.Prim.tagToEnum# @ Bool (GHC.Prim.># x_a173 1)
        of _ [Occ=Dead] {
          False -> Main.f2_go ys_a16a;
          True -> GHC.Types.: @ Int a_aEq (Main.f2_go ys_a16a)
        }
        }
    }
end Rec }


</pre>
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diff.md

What do two different ways of filtering a list compile to?

(Example: take a list of tuples (a,b), and return a list of a for every (a,b) where b > 1.)

`map fst . filter ((>1) . snd)`

`[a | (a,b) <- x, b > 1]`

Rec { Main.f3 [Occ=LoopBreaker] :: forall a_aWS. [(a_aWS, Integer)] -> [a_aWS] Main.f3 = \ (@ a_aWS) (ds_a16M :: [(a_aWS, Integer)]) -> case ds_a16M of _ [Occ=Dead] { [] -> GHC.Types.[] @ a_aWS; : y_a16R ys_a16S -> case y_a16R of _ [Occ=Dead] { (ds1_a17j, y1_a17k) -> case integer-gmp-1.0.0.0:GHC.Integer.Type.gtInteger# y1_a17k Main.f5 of wild2_a18o { __DEFAULT -> case GHC.Prim.tagToEnum# @ Bool wild2_a18o of _ [Occ=Dead] { False -> Main.f3 @ a_aWS ys_a16S; True -> GHC.Types.: @ a_aWS ds1_a17j (Main.f3 @ a_aWS ys_a16S) } } } } end Rec }

f2 :: forall t_aVS a_aVT. (Num a_aVT, Ord a_aVT) => [(t_aVS, a_aVT)] -> [t_aVS] f2 = \ (@ t_aVX) (@ a_aVY) ($dNum_aVZ :: Num a_aVY) ($dOrd_aW0 :: Ord a_aVY) (x_aE5 :: [(t_aVX, a_aVY)]) -> let { lvl_s19c :: a_aVY [LclId, Str=DmdType] lvl_s19c = fromInteger @ a_aVY $dNum_aVZ Main.f5 } in letrec { go2_a16L [Occ=LoopBreaker] :: [(t_aVX, a_aVY)] -> [t_aVX] [LclId, Arity=1, Str=DmdType ] go2_a16L = \ (ds_a16M :: [(t_aVX, a_aVY)]) -> case ds_a16M of _ [Occ=Dead] { [] -> GHC.Types.[] @ t_aVX; : y_a16R ys_a16S -> case y_a16R of _ [Occ=Dead] { (a1_aE6, b_aE7) -> case > @ a_aVY $dOrd_aW0 b_aE7 lvl_s19c of _ [Occ=Dead] { False -> go2_a16L ys_a16S; True -> GHC.Types.: @ t_aVX a1_aE6 (go2_a16L ys_a16S) } } }; } in go2_a16L x_aE5