> I have not met a performance issue yet. Thanks. I'm just a beginner.
> I was just wondering how they will be different. For instance,
>
> safeHead [] = Nothing
> safeHead (x : xs) = Just x
>
> vesus
>
> safeHead (x : xs) = Just x
> safeHead [] = Nothing
There is no difference in this case. GHC compiles that down to the very same
thing.
Given this file:
> safeHead [] = Nothing
> safeHead (x:xs) = Just x
>
> safeHead' (x:xs) = Just x
> safeHead' [] = Nothing
You can look at the ghc core (the intermediate compiler output produced by GHC)
to find out what the difference is. Core is basically like machine-generated
Haskell with some syntactical oddities. Don Steward's ghc-core program on
hackage displays core nicely.
Here are the relevant core blocks:
> safeHead' =
> \ (@ a_acw) (ds_dcU :: [a_acw]) ->
> case ds_dcU of _ {
> [] -> Data.Maybe.Nothing @ a_acw;
> : x_abJ xs_abK -> Data.Maybe.Just @ a_acw x_abJ
> }
> safeHead =
> \ (@ a_acy) (ds_dcX :: [a_acy]) ->
> case ds_dcX of _ {
> [] -> Data.Maybe.Nothing @ a_acy;
> : x_abx xs_aby -> Data.Maybe.Just @ a_acy x_abx
> }
And, well, they're exactly the same (sans a few differences in variable naming,
which have no impact whatsoever.) Your pattern match gets compiled down to a
case pattern match, and the arguments are re-ordered. While you *can* look at
the assembly, from here on out, it doesn't make sense, because GHC uses Core as
a basis for compilation, *not* your code (it does your code as a basis for
Core.)
For completion's sake, the assembly, which is unexcitingly identical in both
cases, is below. Note that the only thing interesting here is that safeHead' got
translated to safeHeadzq, which is… well. Cute, and nothing else.
Really though, if you're still at RWH, chapter three, it's Ok to be interested
in these kinds of subtle performance differences, but you shouldn't concentrate
on them.
At this point, you can trust the compiler to optimize your code enough, and you
can worry about performance if your program is actually slow.
Happy Haskell hacking :-)
Aleks
Main_safeHeadzq_closure:
.quad Main_safeHeadzq_info
.text
.align 8
.quad 0
.quad 32
se2_info:
.Lcek:
movq %rbx,%rax
andq $7,%rax
cmpq $2,%rax
jae .Lcel
movl $base_DataziMaybe_Nothing_closure+1,%ebx
addq $8,%rbp
jmp *0(%rbp)
.Lcel:
addq $16,%r12
cmpq 144(%r13),%r12
ja .Lces
movq $base_DataziMaybe_Just_con_info,-8(%r12)
movq 6(%rbx),%rax
movq %rax,0(%r12)
leaq -6(%r12),%rbx
addq $8,%rbp
jmp *0(%rbp)
.Lces:
movq $16,184(%r13)
.Lceq:
jmp *-16(%r13)
.text
.align 8
.quad 4294967301
.quad 0
.quad 15
Main_safeHead_closure:
.quad Main_safeHead_info
.text
.align 8
.quad 0
.quad 32
seJ_info:
.Lcf1:
movq %rbx,%rax
andq $7,%rax
cmpq $2,%rax
jae .Lcf2
movl $base_DataziMaybe_Nothing_closure+1,%ebx
addq $8,%rbp
jmp *0(%rbp)
.Lcf2:
addq $16,%r12
cmpq 144(%r13),%r12
ja .Lcf9
movq $base_DataziMaybe_Just_con_info,-8(%r12)
movq 6(%rbx),%rax
movq %rax,0(%r12)
leaq -6(%r12),%rbx
addq $8,%rbp
jmp *0(%rbp)
.Lcf9:
movq $16,184(%r13)
.Lcf7:
jmp *-16(%r13)
.text
.align 8
.quad 4294967301
.quad 0
.quad 15
