The compiler dumps are illuminating, thank you.
I'm afraid I don't always compile under -O. In fact I never
debug with -O. I see now what I'm missing.
(Pain, grief, despair.)
Ketil Malde wrote:
>
> On Thu, 2007-08-16 at 12:50 -0700, Kim-Ee Yeoh wrote:
>> Aaron Denney wrote:
>> > The compiler should be able to
>> > assemble it all at compile time, right?
>>
>> 'Course not. The (++) function like all Haskell functions is only a
>> /promise/ to do its job. What does "assembling at compile time"
>> mean here:
>>
>> s = "I will not write infinite loops " ++ s
>
> % cat C.hs
>
> module Test where
>
> x = "Foo" ++ "Bar"
> y = "Zot" ++ y
>
> % ghc -ddump-simpl C.hs
>
> ==================== Tidy Core ====================
> Test.x :: [GHC.Base.Char]
> [GlobalId]
> []
> Test.x =
> GHC.Base.++
> @ GHC.Base.Char (GHC.Base.unpackCString# "Foo")
> (GHC.Base.unpackCString# "Bar")
>
> Rec {
> Test.y :: [GHC.Base.Char]
> [GlobalId]
> []
> Test.y = GHC.Base.++ @ GHC.Base.Char (GHC.Base.unpackCString# "Zot")
> Test.y
> end Rec }
>
> If I interpret it correctly, the compiler does approximately nothing -
> reasonably enough, since we didn't ask for optimization. With -O:
>
> % ghc -ddump-simpl C.hs -O
>
> ==================== Tidy Core ====================
> Rec {
> Test.y :: [GHC.Base.Char]
> [GlobalId]
> [Str: DmdType]
> Test.y = GHC.Base.unpackAppendCString# "Zot" Test.y
> end Rec }
>
> Test.x :: [GHC.Base.Char]
> [GlobalId]
> [Str: DmdType]
> Test.x = GHC.Base.unpackCString# "FooBar"
>
> y gets turned into an unpackAppendCString#, which I can only presume is
> a sensible way to represent a cyclic list, while x gets concatenated
> compile-time.
>
--
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