Thanks! These examples and counterexamples help a lot
in understanding.
Markus
> The reason is that sum only works on integer lists, whereas the given
> type for fun requires that the first argument be a function that works
> on lists of any type, such as length.
> If sum were allowed as argumen
[EMAIL PROTECTED] wrote:
>
> I think I get it. But to be sure:
> The forall means: The type for a may not be the same
> throughout the whole function. It just has to follow
> the pattern specified inside the braces.
Not exactly. In:
> > fun::(forall a.[a]->Int)->[b]->[c]->Int
> > fun f x y = f x
> Interestingly, when I want hugs to show me the type
> of
> > fun::(forall a.[a]->Int)->[b]->[c]->Int
> it tells me: ERROR - Use of fun requires at least 1 argument
> Why that? At least I have explicitely specified the type.
Hmm, ghci behaves properly.
But using hugs I get the same error :-(.
N
I think I get it. But to be sure:
The forall means: The type for a may not be the same
throughout the whole function. It just has to follow
the pattern specified inside the braces.
Interestingly, when I want hugs to show me the type
of
> fun::(forall a.[a]->Int)->[b]->[c]->Int
it tells me: ERROR -
> concrete: What is the difference between
> (forall b. Term b => b -> b) -> a -> a
> and
> (Term b) => (b -> b) -> a -> a
> ?
One may want:
fun f x y = f x + f y
for instance:
fun length [True, False] [1,2]
Therefore, I would say, you need typing a la
fun::(forall a.[a]->Int)->[b]->[c]->
> http://research.microsoft.com/~simonpj/papers/hmap/
The technique you describe in that paper is exactly what I was
wanting for many times. I often stopped using algebraic data types
because of the immense amount of boilerplate I had to introduce.
Thanks!
But now I have a question regarding th
Hal Daume wrote:
> -- *Main> test $ MkFoo (0::Int)
> -- Just True
> -- *Main> test $ MkBar 'a'
> -- Just True
> i forgot to mention the constraint
> that i don't want the user to have to use the MkFoo/MkBar
> constructors. if i could use them internally to 'test', that would be
> great, bu
TECTED]
> | Sent: 21 March 2003 04:19
> | To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
> | Subject: Re: simulating dynamic dispatch
> |
> |
> | > i'm hoping to be able to simulate a sort of dynamic dispatch based
> on
> | > class instances.
> |
> | It se
I think you are looking like something done in the
HTML-Combinator-libraries.
I tried something like that some time ago, but didn't finish. But perhaps
you
can get an idea from that. See the files with this mail. (The code is in
unknown condition.)
Markus
> ah, yes. i was aware that would work.
| Sent: 21 March 2003 04:19
| To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
| Subject: Re: simulating dynamic dispatch
|
|
| > i'm hoping to be able to simulate a sort of dynamic dispatch based
on
| > class instances.
|
| It seems you want to dispatch based not on a type but on the
|
On 20-Mar-2003, Hal Daume III <[EMAIL PROTECTED]> wrote:
> i'm hoping to be able to simulate a sort of dynamic dispatch based on
> class instances. basically a function which takes a value and depending
> on what classes it is an instance of, does something.
I call this feature "dynamic type clas
> -- *Main> test $ MkFoo (0::Int)
> -- Just True
> -- *Main> test $ MkFoo (10::Int)
> -- Just False
> -- *Main> test $ MkBar 'a'
> -- Just True
> -- *Main> test $ MkBar 'b'
> -- Just False
ah, yes. i was aware that would work. i forgot to mention the constraint
that i don't want the user t
> i'm hoping to be able to simulate a sort of dynamic dispatch based on
> class instances.
It seems you want to dispatch based not on a type but on the
constraint of a type.
You code almost worked. Here's the a bit updated and working version.
class Foo a where { foo :: a -> Bool }
class Bar a
hi all,
i'm hoping to be able to simulate a sort of dynamic dispatch based on
class instances. basically a function which takes a value and depending
on what classes it is an instance of, does something. a simple example
might be a 'maybeShow' function which sematically looks like:
> maybeShow
14 matches
Mail list logo
