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Today's Topics:
1. Error message: "Couldn't match expected type" (S. H. Aegis)
2. Re: Error message: "Couldn't match expected type"
(Brandon Allbery)
3. Re: Error message: "Couldn't match expected type" (S. H. Aegis)
----------------------------------------------------------------------
Message: 1
Date: Fri, 3 Oct 2014 01:40:30 +0900
From: "S. H. Aegis" <[email protected]>
To: "[email protected]" <[email protected]>
Subject: [Haskell-beginners] Error message: "Couldn't match expected
type"
Message-ID:
<cajp-nqy8srtndymuemh4dhg2w83fukxtp1ywdup3nida2uw...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hello.
I'm learning Haskell on OSX.
How can I fix this error message?
(Code from The Haskell School of Expression, P40)
Sorry for border you...
Sok.
import Graphics.SOE
main() =
runGraphics (
do w <- openWindow "My First Graphics Program" (300, 300)
drawInWindow w (text(100, 200) "HelloFraphicsWorld")
spaceClose w
)
spaceClose :: Window -> IO()
spaceClose w
= do k <- getKey w
if k == ' ' then closeWindow w
else spaceClose w
[1 of 1] Compiling Main ( firstGraphics.hs, firstGraphics.o )
firstGraphics.hs:3:1:
Couldn't match expected type `IO t0' with actual type `() -> IO ()'
In the expression: main
When checking the type of the function `main'
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Message: 2
Date: Thu, 2 Oct 2014 12:49:56 -0400
From: Brandon Allbery <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Error message: "Couldn't match
expected type"
Message-ID:
<cakfcl4ujlrqfaazlrz582jmj+1gcovyr47pvieoxwhzngcj...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Thu, Oct 2, 2014 at 12:40 PM, S. H. Aegis <[email protected]> wrote:
> main() =
You have declared (well, described, but by type inference that's the same
thing) main to take a parameter of type "unit" (empty tuple). main doesn't
take parameters.
Haskell parameters do not work the way most common languages do; using
parentheses in function calls the way you would in C/Java/Python etc. will
generally get you unexpected type errors, because you're telling it you're
passing tuples around. So, the function call
f() (f :: () -> a)
is different from
f (f :: a)
and the function call
f(a, b) (f :: (a,b) -> c)
is different from
f a b (f :: a -> b -> c)
(The inferred type of `f` is shown after each call.)
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 3
Date: Fri, 3 Oct 2014 01:55:40 +0900
From: "S. H. Aegis" <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Error message: "Couldn't match
expected type"
Message-ID:
<cajp-nqzt9ef5rqtjhdjcoueuvjkxlqub8qegz-_o9jyixjj...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Oh... my...
Thank you so much.
Maybe, I'm confused with C.
Fix main() to main, it works! =)
Thank you again, and have a nice day.
Sincerely, Sok
2014-10-03 1:49 GMT+09:00 Brandon Allbery <[email protected]>:
> On Thu, Oct 2, 2014 at 12:40 PM, S. H. Aegis <[email protected]> wrote:
>
>> main() =
>
>
> You have declared (well, described, but by type inference that's the same
> thing) main to take a parameter of type "unit" (empty tuple). main doesn't
> take parameters.
>
> Haskell parameters do not work the way most common languages do; using
> parentheses in function calls the way you would in C/Java/Python etc. will
> generally get you unexpected type errors, because you're telling it you're
> passing tuples around. So, the function call
>
> f() (f :: () -> a)
>
> is different from
>
> f (f :: a)
>
> and the function call
>
> f(a, b) (f :: (a,b) -> c)
>
> is different from
>
> f a b (f :: a -> b -> c)
>
> (The inferred type of `f` is shown after each call.)
>
> --
> brandon s allbery kf8nh sine nomine
> associates
> [email protected]
> [email protected]
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
>
--
Sok Ha, CHANG
Dr. Chang's Clinic. #203. 503-23. AmSa-Dong, GangDong-Gu, Seoul.
Tel: +82-2-442-7585
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