Re: [Haskell-beginners] GHCi bug?

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GHCi bug?

Ozgur Akgun

16 Nov 2010 16 Nov '10

11:44 a.m.

Unfortunately, yes. On 16 November 2010 11:39, Russ Abbott wrote:

...

I'm not sure what you are saying here. Are you saying that it's not a bug that GHCi went into a coma when I file was loaded and a prompt entered? * -- Russ* *** * **

On Tue, Nov 16, 2010 at 3:34 AM, Ozgur Akgun wrote:

...
On 16 November 2010 05:12, Russ Abbott wrote:

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I know the code isn't correct. My point is that the compiler didn't complain when the code was loaded, and the interpreter died when it was executed. That shouldn't happen. * -- Russ*

It would be *cool* for GHC to make an analysis about the instance methods of a type class, and their default implementations to find the minimal subset of methods you have to implement to get a valid (for some definition of valid) instance declaration. But it doesn't do such a thing. And it is not an easy task.

GHC only warns you if you do not implement a method which doesn't have a default implementation. In your example, this is not the case.

Best,

-- Ozgur Akgun

-- Ozgur Akgun

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Thomas Davie

16 Nov 16 Nov

12:17 p.m.

New subject: GHCi bug?

To explain that further as to why it's not a bug... GHCi's job is to run the code you enter. You asked it to show a value, so it executed the show function (correctly). The default implementation called showPrec, which invoked show, etc. There may well be a missing feature here (to check of you implement nothing other than the defaults which are very likely to loop), but not a bug. Bob On 16 Nov 2010, at 11:44, Ozgur Akgun wrote:

...

Unfortunately, yes.

On 16 November 2010 11:39, Russ Abbott wrote: I'm not sure what you are saying here. Are you saying that it's not a bug that GHCi went into a coma when I file was loaded and a prompt entered?

-- Russ

On Tue, Nov 16, 2010 at 3:34 AM, Ozgur Akgun wrote:

On 16 November 2010 05:12, Russ Abbott wrote: I know the code isn't correct. My point is that the compiler didn't complain when the code was loaded, and the interpreter died when it was executed. That shouldn't happen.

-- Russ

It would be cool for GHC to make an analysis about the instance methods of a type class, and their default implementations to find the minimal subset of methods you have to implement to get a valid (for some definition of valid) instance declaration. But it doesn't do such a thing. And it is not an easy task.

GHC only warns you if you do not implement a method which doesn't have a default implementation. In your example, this is not the case.

Best,

-- Ozgur Akgun

-- Ozgur Akgun _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Ozgur Akgun

1:10 p.m.

New subject: GHCi bug?

On 16 November 2010 12:17, Thomas Davie wrote:

...

GHCi's job is to run the code you enter.

Likewise, in many other languages when you write the following you'll get an infinite rule. And it just compiles (or gets interpreted) fine. while(true) { } Just another perspective. Ozgur

Bastian Erdnüß

8:37 p.m.

New subject: GHCi bug?

I didn't test the OP's example. But if I do let x = y ; y = x in GHCi and then run 'x', i can interrupt it with ^C. As I understand the bug is that you cannot interrupt the infinite loop in this special case. Btw. I thought instance Show Test where -- empty body should not work. An instance with no declaration should be just instance Show Test -- no body Maybe the EOF after the 'where' has something todo with this "bug". Bastian On Nov 16, 2010, at 13:17, Thomas Davie wrote:

...

To explain that further as to why it's not a bug...

GHCi's job is to run the code you enter. You asked it to show a value, so it executed the show function (correctly). The default implementation called showPrec, which invoked show, etc.

There may well be a missing feature here (to check of you implement nothing other than the defaults which are very likely to loop), but not a bug.

Bob

On 16 Nov 2010, at 11:44, Ozgur Akgun wrote:

...
Unfortunately, yes.

On 16 November 2010 11:39, Russ Abbott wrote: I'm not sure what you are saying here. Are you saying that it's not a bug that GHCi went into a coma when I file was loaded and a prompt entered?

-- Russ

On Tue, Nov 16, 2010 at 3:34 AM, Ozgur Akgun wrote:

On 16 November 2010 05:12, Russ Abbott wrote: I know the code isn't correct. My point is that the compiler didn't complain when the code was loaded, and the interpreter died when it was executed. That shouldn't happen.

-- Russ

It would be cool for GHC to make an analysis about the instance methods of a type class, and their default implementations to find the minimal subset of methods you have to implement to get a valid (for some definition of valid) instance declaration. But it doesn't do such a thing. And it is not an easy task.

GHC only warns you if you do not implement a method which doesn't have a default implementation. In your example, this is not the case.

Best,

-- Ozgur Akgun

-- Ozgur Akgun _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Bastian Erdnüß
Ozgur Akgun
Thomas Davie