"Found hole"

Volker Wysk

20 Jan 2015 20 Jan '15

1:36 p.m.

Hello! What is a "hole"? This program fails to compile: main = _exit 0 I get this error message: ex.hs:1:8: Found hole ‘_exit’ with type: t Where: ‘t’ is a rigid type variable bound by the inferred type of main :: t at ex.hs:1:1 Relevant bindings include main :: t (bound at ex.hs:1:1) In the expression: _exit In an equation for ‘main’: main = _exit When I replace "_exit" with "foo", it produces a "not in scope" error, as expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries. Bye Volker

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Brandon Allbery

20 Jan 20 Jan

1:44 p.m.

On Tue, Jan 20, 2015 at 1:36 PM, Volker Wysk wrote:

...

What is a "hole"?

https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/typed-holes.... When I replace "_exit" with "foo", it produces a "not in scope" error, as

...

expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries.

The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first line of the source file. (I am not sure why this extension was enabled by default.) -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

Volker Wysk

2:22 p.m.

Hi! Am Dienstag, 20. Januar 2015, 13:44:01 schrieben Sie:

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The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first line of the source file.

I get this error, when I use "{-# LANGUAGE NoTypedHoles #-}": ex.hs:1:14: Unsupported extension: NoTypedHoles I'm using GHC 7.8.3. Could it be that the "NoTypedHoles" extension was added not before 7.8.4 (the most current version). Bye Volker

Alex Hammel

2:25 p.m.

The only reference to a NoTypedHoles extension google can find is this thread. Odd. On Tue, Jan 20, 2015 at 11:22 AM, Volker Wysk wrote:

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Hi!

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The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first

Am Dienstag, 20. Januar 2015, 13:44:01 schrieben Sie: line

...
of the source file.

I get this error, when I use "{-# LANGUAGE NoTypedHoles #-}":

ex.hs:1:14: Unsupported extension: NoTypedHoles

I'm using GHC 7.8.3. Could it be that the "NoTypedHoles" extension was added not before 7.8.4 (the most current version).

Bye Volker _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Alex Hammel

3:39 p.m.

You can get typed holes to compile with a warning and a runtime error with the -fdefer-type-errors flag, if that's what you want. However, it's perfectly legal to use identifiers which look like typed holes. This works fine on 7.8.3ghc: _exit = print main = _exit 0 On Tue, Jan 20, 2015 at 11:25 AM, Alex Hammel wrote:

...

The only reference to a NoTypedHoles extension google can find is this thread. Odd.

On Tue, Jan 20, 2015 at 11:22 AM, Volker Wysk wrote:

...
Hi!

...
The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first

Am Dienstag, 20. Januar 2015, 13:44:01 schrieben Sie: line

...
of the source file.

I get this error, when I use "{-# LANGUAGE NoTypedHoles #-}":

ex.hs:1:14: Unsupported extension: NoTypedHoles

I'm using GHC 7.8.3. Could it be that the "NoTypedHoles" extension was added not before 7.8.4 (the most current version).

Bye Volker _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

migmit

3:51 p.m.

DON'T DO THAT! Seriously, turn off compile-time type checking completely just to start an identifier with an underscore??? Отправлено с iPad

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20 янв. 2015 г., в 21:39, Alex Hammel написал(а):

You can get typed holes to compile with a warning and a runtime error with the -fdefer-type-errors flag, if that's what you want.

However, it's perfectly legal to use identifiers which look like typed holes. This works fine on 7.8.3ghc:

_exit = print main = _exit 0

...
On Tue, Jan 20, 2015 at 11:25 AM, Alex Hammel wrote: The only reference to a NoTypedHoles extension google can find is this thread. Odd.

...
On Tue, Jan 20, 2015 at 11:22 AM, Volker Wysk wrote: Hi!

Am Dienstag, 20. Januar 2015, 13:44:01 schrieben Sie:

...
The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first line of the source file.

I get this error, when I use "{-# LANGUAGE NoTypedHoles #-}":

ex.hs:1:14: Unsupported extension: NoTypedHoles

I'm using GHC 7.8.3. Could it be that the "NoTypedHoles" extension was added not before 7.8.4 (the most current version).

Bye Volker _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

David Feuer

4:01 p.m.

Just use exit_ or something instead. Typed holes are a *really useful* mechanism. On Tue, Jan 20, 2015 at 3:51 PM, migmit wrote:

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DON'T DO THAT!

Seriously, turn off compile-time type checking completely just to start an identifier with an underscore???

Отправлено с iPad

20 янв. 2015 г., в 21:39, Alex Hammel написал(а):

You can get typed holes to compile with a warning and a runtime error with the -fdefer-type-errors flag, if that's what you want.

However, it's perfectly legal to use identifiers which look like typed holes. This works fine on 7.8.3ghc:

_exit = print main = _exit 0

On Tue, Jan 20, 2015 at 11:25 AM, Alex Hammel wrote:

...
The only reference to a NoTypedHoles extension google can find is this thread. Odd.

On Tue, Jan 20, 2015 at 11:22 AM, Volker Wysk wrote:

...
Hi!

Am Dienstag, 20. Januar 2015, 13:44:01 schrieben Sie:

...
The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first line of the source file.

I get this error, when I use "{-# LANGUAGE NoTypedHoles #-}":

ex.hs:1:14: Unsupported extension: NoTypedHoles

I'm using GHC 7.8.3. Could it be that the "NoTypedHoles" extension was added not before 7.8.4 (the most current version).

Bye Volker _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

htebalaka

6:11 p.m.

Unless it behaves differently in GHC than GHCi, you can still use underscore prefixed identifiers, provided they are in scope. I only get a type hole message if the identifier isn't defined anywhere else.

...

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let _x = 2 _x 2 _y Found hole '_y' with type: t ...

-- View this message in context: http://haskell.1045720.n5.nabble.com/Found-hole-tp5764054p5764081.html Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.

Merijn Verstraaten

21 Jan 21 Jan

5:03 a.m.

Typed holes is not an extension, because it's considered a warning/error. The reason for this is that code with typed holes is NOT valid haskell to begin with, therefore the behaviour doesn't conflict with any description in the report. Additionally, the ability to disable the typed holes warning is disappearing in 7.10 (well, sorta, you can silence them, but you can't get the "old" error message back). If there's any comments on how to improve the warning message to be less confusing I'd be interested to hear them. Cheers, Merijn

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On 20 Jan 2015, at 19:44, Brandon Allbery wrote:

On Tue, Jan 20, 2015 at 1:36 PM, Volker Wysk wrote: What is a "hole"?

https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/typed-holes....

When I replace "_exit" with "foo", it produces a "not in scope" error, as expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries.

The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first line of the source file. (I am not sure why this extension was enabled by default.)

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Volker Wysk

9:11 a.m.

Am Mittwoch, 21. Januar 2015, 11:03:38 schrieben Sie:

...

If there's any comments on how to improve the warning message to be less confusing I'd be interested to hear them.

What about "Found hole _foo with type: bar. This can also mean that _foo is not in scope". Bye V.W.

David Feuer

9:42 a.m.

If such verbiage is added, it should probably read more like "If you did not intend to insert a typed hole, _foo may have been misspelled." On Jan 21, 2015 9:11 AM, "Volker Wysk" wrote:

...

Am Mittwoch, 21. Januar 2015, 11:03:38 schrieben Sie:

...
If there's any comments on how to improve the warning message to be less confusing I'd be interested to hear them.

What about "Found hole _foo with type: bar. This can also mean that _foo is not in scope".

Bye V.W. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Volker Wysk

10:25 a.m.

Am Mittwoch, 21. Januar 2015, 09:42:05 schrieben Sie:

...

If such verbiage is added, it should probably read more like "If you did not intend to insert a typed hole, _foo may have been misspelled."

What about "If you did not intend to insert a typed hole, _foo may have been misspelled or out of scope." In my case, it has been out of scope, but not misspelled. I think verbiage is a good thing in this case. Bye V.W.

Stephen Paul Weber

9:55 a.m.

...

Typed holes is not an extension, because it's considered a warning/error. The reason for this is that code with typed holes is NOT valid haskell to begin with, therefore the behaviour doesn't conflict with any description in the report.

Well, now I feel very silly about my last email to this list. This is what I understood. I'm glad to see this is indeed the case.

Stephen Paul Weber

9:53 a.m.

...

The leading underscore invokes the typed holes extension. If you want to use such names, you'll need {-# LANGUAGE NoTypedHoles #-} as the first line of the source file. (I am not sure why this extension was enabled by default.)

This is very concening for me. Extensions should *never* be enabled by default! When the TypedHoles discussion originally came up there was definitely talk of using a switch to enable them (in fact, IIRC, they weren't originally really quite an extension, just a compiler mode, but that may have been a misinterpretation on my part). Having them on by default mean that valid Haskell2010 programs might get rejected by GHC by default, which is a pretty bad state of affairs. -- Stephen Paul Weber, @singpolyma See http://singpolyma.net for how I prefer to be contacted edition right joseph

David Feuer

10:11 a.m.

On Jan 21, 2015 9:53 AM, "Stephen Paul Weber" wrote:

...

Having them on by default mean that valid Haskell2010 programs might get rejected by GHC by default, which is a pretty bad state of affairs.

It would be if it were true. But it's not. All that changes is that you get different error messages in some cases.

Brandon Allbery

11:42 a.m.

On Wed, Jan 21, 2015 at 9:53 AM, Stephen Paul Weber < singpolyma@singpolyma.net> wrote:

...

This is very concening for me. Extensions should *never* be enabled by default!

If you read on, you'll find that I was working from an older proposal that was never implemented. It is instead a modified version of a normal error message, and the syntax used ensures that it is always an error anyway, since it can only be invoked with either illegal expression syntax "_" or an *undefined* name "_foo"). -- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

Edward Z. Yang

20 Jan 20 Jan

1:46 p.m.

Hello Volker, All identifiers prefixed with an underscore are "typed holes", see: https://downloads.haskell.org/~ghc/7.8.3/docs/html/users_guide/typed-holes.h... Edward Excerpts from Volker Wysk's message of 2015-01-20 10:36:09 -0800:

...

Hello!

What is a "hole"?

This program fails to compile:

main = _exit 0

I get this error message:

ex.hs:1:8: Found hole ‘_exit’ with type: t Where: ‘t’ is a rigid type variable bound by the inferred type of main :: t at ex.hs:1:1 Relevant bindings include main :: t (bound at ex.hs:1:1) In the expression: _exit In an equation for ‘main’: main = _exit

When I replace "_exit" with "foo", it produces a "not in scope" error, as expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries.

Bye Volker

htebalaka

1:47 p.m.

They are described at these two links: https://www.haskell.org/haskellwiki/GHC/Typed_holes https://downloads.haskell.org/~ghc/7.8.1-rc1/docs/html/users_guide/typed-hol... Essentially, identifiers that are not otherwise in scope and consist of an underscore or that have a trailing underscore are treated as holes, for which you wish to know which type was inferred. Previously you would need to do something like add a wrong type signature, so that the compiler would complain that the type you gave doesn't match what it inferred. Though I get a different error message: Found hole ‘_exit’ with type: a0 -> t Where: ‘a0’ is an ambiguous type variable ‘t’ is a rigid type variable bound by There really should be a Num constraint as well, but at least in 7.8 it doesn't seem to include those. Volker Wysk-5 wrote

...

Hello!

What is a "hole"?

This program fails to compile:

main = _exit 0

I get this error message:

ex.hs:1:8: Found hole ‘_exit’ with type: t Where: ‘t’ is a rigid type variable bound by the inferred type of main :: t at ex.hs:1:1 Relevant bindings include main :: t (bound at ex.hs:1:1) In the expression: _exit In an equation for ‘main’: main = _exit

When I replace "_exit" with "foo", it produces a "not in scope" error, as expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries.

Bye Volker

_______________________________________________ Glasgow-haskell-users mailing list

...

Glasgow-haskell-users@

...

http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

-- View this message in context: http://haskell.1045720.n5.nabble.com/Found-hole-tp5764054p5764057.html Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.

Edward Kmett

6:49 p.m.

FWIW- you can think of a 'hole' as a "not in scope" error with a ton of useful information about the type such a term would have to have in order to go in the location you referenced it. This promotes a very useful style of type-driven development that is common in Agda, where you write out your program and then leave holes that start with _'s to fill in later. Since no new programs are accepted or rejected, GHC turned on "holes" support by default. Users have historically faked this style with ImplicitParams by labeling their holes ?foo, but that approach doesn't give any information on what local stuff could be used to plug the hole. The only thing that changed is the nature of the error message, which carefully track what variables are in scope, how they are instantiated, and what type the missing term is used at. Since libraries like lens were already making heavy use of the _'d namespace this only happens if the name isn't already in use. This is why you can define _'d things just fine. The main thing that happened is if you typo the name of a lens, well, your error messages got even worse. ;) -Edward On Tue, Jan 20, 2015 at 1:36 PM, Volker Wysk wrote:

...

Hello!

What is a "hole"?

This program fails to compile:

main = _exit 0

I get this error message:

ex.hs:1:8: Found hole ‘_exit’ with type: t Where: ‘t’ is a rigid type variable bound by the inferred type of main :: t at ex.hs:1:1 Relevant bindings include main :: t (bound at ex.hs:1:1) In the expression: _exit In an equation for ‘main’: main = _exit

When I replace "_exit" with "foo", it produces a "not in scope" error, as expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries.

Bye Volker

_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Volker Wysk

21 Jan 21 Jan

2:20 a.m.

New subject: [solved] Re: "Found hole"

Hello! I've found what went wrong: "_exit" wasn't in scope, so it was interpreted to be a "typed hole". Thanks Volker

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Alex Hammel
Brandon Allbery
David Feuer
Edward Kmett
Edward Z. Yang
htebalaka
Merijn Verstraaten
migmit
Stephen Paul Weber
Volker Wysk

"Found hole"

Volker Wysk

Volker Wysk

Alex Hammel

Alex Hammel

Volker Wysk

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