[Haskell-cafe] Python is lazier than Haskell

[Alexander Solla]

On Thu, Apr 28, 2011 at 1:18 PM, Dan Doel <dan.doel at gmail.com> wrote:

> (Sorry if you get this twice, Ertugrul; and if I reply to top. I'm
> stuck with the gmail interface and I'm not used to it.)
>
> On Thu, Apr 28, 2011 at 11:27 AM, Ertugrul Soeylemez <es at ertes.de> wrote:
> > I don't see any problem with this.  Although I usually have a bottom-up
> > approach, so I don't do this too often, it doesn't hurt, when I have to.
>
> I do. It's low tech and inconvenient.
>
> Whenever I program in Haskell, I miss Agda's editing features, where I
> can write:
>
>    foo : Signature
>    foo x y z = ?
>
> Then compile the file. The ? stands in for a term of any type, and
> becomes a 'hole' in my code. The editing environment will then tell me
> what type of term I have to fill into the hole, and give me
> information on what is available in the scope. Then I can write:
>
>
You can simulate that with a type class.  The "no instance" error
substitutes for the "term".

class Hole obj where hole :: obj

foo :: Blah
foo x y z = hole

"No instance for type Blah"

Presumably, you wouldn't want to make instances.  I use a similar construct
for monic and epic functions, since the natural ones tend to be unique
enough.
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