Re: Why are there no Show instances for internal types

[Joachim Breitner]

Hi,

Am Montag, den 20.03.2017, 12:51 + schrieb Simon Peyton Jones via ghc-devs:
> Show and Outputable have very different goals though.
> 
> Really? What's wrong with using Outputable, plus, as Joachim says, 
> 
>   showSDocUnsafe . ppr :: Outputable a => a -> String
> 
> Maybe you want to really really see the precise data constructors
> used.  But for the most part the Outputable instance tells you that,
> but much more legibly.

and if using Outputable is a bit annoying sometimes (e.g. I found it
hard to get the IdInfo of an Id that is not part of a binder, if I
recall correctly, because the default instance does not include that
info, and the pretty printer for “id with idinfo” is not exported),
well, we can always improve that by adding a few more helpful functions
to Outputable.

Greetings,
Joachim

-- 
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RE: Why are there no Show instances for internal types

[Simon Peyton Jones via ghc-devs]

| > >- Is there a reason that there are no derived 'Show' instances for
| > >most types?
| >
| > As Richard mentioned, we don't derive Show due to code size and
| compilation time concerns.

| > Show in particular is rather expensive to derive and seeing as we
| already have Outputable I don't it would make sense to derive it by
| default.
| 
| Show and Outputable have very different goals though.

Really? What's wrong with using Outputable, plus, as Joachim says, 

showSDocUnsafe . ppr :: Outputable a => a -> String

Maybe you want to really really see the precise data constructors used.  But 
for the most part the Outputable instance tells you that, but much more legibly.

Simon
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Re: Why are there no Show instances for internal types

[Ben Gamari]

Tom Sydney Kerckhove  writes:

> On 19-03-17 02:08:56, Rahul Muttineni wrote:
>> Syd, can you tell us what kind of things you were trying to print out?
>
> Maybe I wasn't very clear.
> I'm trying to visualise the internal structure of some of the
> typechecker's output.
> I specifically do NOT need to see the output of Outputable's functions.
> They show the human-readibly version and not the internal structure.
>
Indeed I am sympathetic to this request. In my time working on GHC I
have written raw several variants of `Type -> SDoc`, each exposing
various levels of detail. These are handy and can be a good way to gain
insight into the AST, but I feel like it is hard to come up with
something that is generally applicable; I find each time I need to
expose slightly different details about the internal structure of the
representation.

Cheers,

- Ben



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Re: Why are there no Show instances for internal types

[Tom Sydney Kerckhove]

On 18-03-17 16:13:52, Ben Gamari wrote:
> 
> 
> On March 18, 2017 9:03:48 AM EDT, Tom Sydney Kerckhove 
>  wrote:
> 
> Snip.
> >
> >My questions for you:
> >
> >- Is there a reason that there are no derived 'Show' instances for most
> >  types?
> 
> As Richard mentioned, we don't derive Show due to code size and compilation 
> time concerns.

Okay.

> Show in particular is rather expensive to derive and seeing as we already 
> have Outputable I don't it would make sense to derive it by default.

Show and Outputable have very different goals though.

> I would really like to avoid introducing more CPP into the code base for this 
> particular problem.

Fair enough.

> One alternative which will work in many cases is to simply derive Show 
> yourself using StandaloneDeriving. Does this help?

That doesn't work if some type doesn't have the constructors exposed.
I tried this already, and it would be a good solution if all
constructors were exposed, ...

> Cheers,
> 
> - Ben 
> 
> -- 
> Sent from my Android device with K-9 Mail. Please excuse my brevity.

-- 
Tom Sydney Kerckhove


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Re: Why are there no Show instances for internal types

[Tom Sydney Kerckhove]

On 19-03-17 02:08:56, Rahul Muttineni wrote:
> Syd, can you tell us what kind of things you were trying to print out?

Maybe I wasn't very clear.
I'm trying to visualise the internal structure of some of the
typechecker's output.
I specifically do NOT need to see the output of Outputable's functions.
They show the human-readibly version and not the internal structure.

Does that answer your question?
> Hope that helps,
> Rahul
> 
> On Sun, Mar 19, 2017 at 1:14 AM, Edward Z. Yang  wrote:
> 
> > We can't add Show instances for these types because many types
> > below them, e.g., Type, are cyclic, and would result in infinite
> > output.
> >
> > Perhaps we can add a new type class which a) faithfully represents
> > the Haskell syntax, but b) can deal with cyclic data.  I think that's
> > something people would like (extra compilation time not withstanding).
> > But it sounds annoying to do since the deriving mechanism is not going
> > to help you.
> >
> > Edward
> >
> > Excerpts from Tom Sydney Kerckhove's message of 2017-03-18 14:03:48 +0100:
> > > Dear GHC Devs,
> > >
> > > I am trying to use GHC as a library but I'm having a lot of trouble with
> > > understanding what everything means.
> > > Up to now, I have been able to figure out what to do by reading the
> > > sources, but it ocured to me that much of my struggles could have been
> > > mitigated if the relevant types had Show instances.
> > >
> > > I am specifically talking about the types concerning type checking.
> > > TypecheckedModule and everything below that.
> > > I am aware that most of the types have an Outputable instance, but
> > > there are two problems with that:
> > >
> > > - 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)
> > > - These instances are not intended to show the internal structure of a
> > >   value, but rather a 'human readable' representation of a value.
> > >
> > > My questions for you:
> > >
> > > - Is there a reason that there are no derived 'Show' instances for most
> > >   types?
> > > - Would you accept a diff that adds these?
> > >
> > > Thank you for your time.
> > >
> > ___
> > ghc-devs mailing list
> > ghc-devs@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >
> 
> 
> 
> -- 
> Rahul Muttineni

-- 
Tom Sydney Kerckhove


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Re: Why are there no Show instances for internal types

[Ben Gamari]

Rahul Muttineni  writes:

> I think another way to go about this problem is to figure out an
> alternative to baking in DynFlags to SDocContext (which I feel is the core
> problem here). The only use of those DynFlags is via sdocWithDynFlags and
> 94 call sites use them.
>
Indeed, I would love to see this happen. This exact request is being
tracked as #10143.

Cheers,

- Ben


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Re: Why are there no Show instances for internal types

[Joachim Breitner]

Am Samstag, den 18.03.2017, 14:03 +0100 schrieb Tom Sydney Kerckhove:
> 
> - 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)


You can often get away with

showSDocUnsafe . ppr :: Outputable a => a -> String

Greetings,
Joachim
-- 
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Re: Why are there no Show instances for internal types

[Rahul Muttineni]

I think another way to go about this problem is to figure out an
alternative to baking in DynFlags to SDocContext (which I feel is the core
problem here). The only use of those DynFlags is via sdocWithDynFlags and
94 call sites use them.

- In the frontend, it's used to check for the presence of the flag (like
suppress module prefixes, etc.)
- In the code generator, it's used to get the word size, endianness, and
other platform specific stuff for platform-specific printing.
- Backpack-related stuff needed to get the package state
  - Used in exactly 2 cases: Outputable instances for ComponentId and
InstalledUnitId

>From what I observed with the majority of use cases, sdocWithDynFlags is
used to obviate the need for passing dflags to various ppr* functions,
which is a good idea since without it, we'd probably have to pass around
DynFlags to a whole lot more pure functions throughout the codebase.

So as others have said, Show instances are just not practical because
printing many of the GHC types is highly dependent on the platform and what
flags GHC was invoked with.

There are three solutions here:
1.) Figure out a subset of DynFlags (flags, platform details, package
state) and only allow those inside of SDocContext and extend SDocContext as
 new use cases come up. This is probably not practical as it would
require sweeping changes.
2.) Provide a stock set of DynFlags for the purpose of printing with
Outputable. It's easy to do for flags and platform details, but tricky to
do for package state. This seems to be the most reasonable solution if some
sane substitute for package state can be used.

Syd, can you tell us what kind of things you were trying to print out? You
can try to pass in unsafeGlobalDynFlags but it may not be what you want. It
gets written to on the initialisation of the GHC monad and after the
command line options are parsed (so everything will be properly initialised
except for package state).

Hope that helps,
Rahul

On Sun, Mar 19, 2017 at 1:14 AM, Edward Z. Yang  wrote:

> We can't add Show instances for these types because many types
> below them, e.g., Type, are cyclic, and would result in infinite
> output.
>
> Perhaps we can add a new type class which a) faithfully represents
> the Haskell syntax, but b) can deal with cyclic data.  I think that's
> something people would like (extra compilation time not withstanding).
> But it sounds annoying to do since the deriving mechanism is not going
> to help you.
>
> Edward
>
> Excerpts from Tom Sydney Kerckhove's message of 2017-03-18 14:03:48 +0100:
> > Dear GHC Devs,
> >
> > I am trying to use GHC as a library but I'm having a lot of trouble with
> > understanding what everything means.
> > Up to now, I have been able to figure out what to do by reading the
> > sources, but it ocured to me that much of my struggles could have been
> > mitigated if the relevant types had Show instances.
> >
> > I am specifically talking about the types concerning type checking.
> > TypecheckedModule and everything below that.
> > I am aware that most of the types have an Outputable instance, but
> > there are two problems with that:
> >
> > - 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)
> > - These instances are not intended to show the internal structure of a
> >   value, but rather a 'human readable' representation of a value.
> >
> > My questions for you:
> >
> > - Is there a reason that there are no derived 'Show' instances for most
> >   types?
> > - Would you accept a diff that adds these?
> >
> > Thank you for your time.
> >
> ___
> ghc-devs mailing list
> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>



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Rahul Muttineni
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Re: Why are there no Show instances for internal types

[Ben Gamari]

On March 18, 2017 9:03:48 AM EDT, Tom Sydney Kerckhove 
 wrote:

Snip.
>
>My questions for you:
>
>- Is there a reason that there are no derived 'Show' instances for most
>  types?

As Richard mentioned, we don't derive Show due to code size and compilation 
time concerns. Show in particular is rather expensive to derive and seeing as 
we already have Outputable I don't it would make sense to derive it by default. 
I would really like to avoid introducing more CPP into the code base for this 
particular problem.

One alternative which will work in many cases is to simply derive Show yourself 
using StandaloneDeriving. Does this help?

Cheers,

- Ben 

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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Re: Why are there no Show instances for internal types

[Alan & Kim Zimmerman]

And I guess it would be bad to use Show, but make custom instances for the
problematic types that did not loop?

Alan

On 18 Mar 2017 9:44 pm, "Edward Z. Yang"  wrote:

> We can't add Show instances for these types because many types
> below them, e.g., Type, are cyclic, and would result in infinite
> output.
>
> Perhaps we can add a new type class which a) faithfully represents
> the Haskell syntax, but b) can deal with cyclic data.  I think that's
> something people would like (extra compilation time not withstanding).
> But it sounds annoying to do since the deriving mechanism is not going
> to help you.
>
> Edward
>
> Excerpts from Tom Sydney Kerckhove's message of 2017-03-18 14:03:48 +0100:
> > Dear GHC Devs,
> >
> > I am trying to use GHC as a library but I'm having a lot of trouble with
> > understanding what everything means.
> > Up to now, I have been able to figure out what to do by reading the
> > sources, but it ocured to me that much of my struggles could have been
> > mitigated if the relevant types had Show instances.
> >
> > I am specifically talking about the types concerning type checking.
> > TypecheckedModule and everything below that.
> > I am aware that most of the types have an Outputable instance, but
> > there are two problems with that:
> >
> > - 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)
> > - These instances are not intended to show the internal structure of a
> >   value, but rather a 'human readable' representation of a value.
> >
> > My questions for you:
> >
> > - Is there a reason that there are no derived 'Show' instances for most
> >   types?
> > - Would you accept a diff that adds these?
> >
> > Thank you for your time.
> >
> ___
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> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
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Re: Why are there no Show instances for internal types

[Edward Z. Yang]

We can't add Show instances for these types because many types
below them, e.g., Type, are cyclic, and would result in infinite
output.

Perhaps we can add a new type class which a) faithfully represents
the Haskell syntax, but b) can deal with cyclic data.  I think that's
something people would like (extra compilation time not withstanding).
But it sounds annoying to do since the deriving mechanism is not going
to help you.

Edward

Excerpts from Tom Sydney Kerckhove's message of 2017-03-18 14:03:48 +0100:
> Dear GHC Devs,
> 
> I am trying to use GHC as a library but I'm having a lot of trouble with
> understanding what everything means.
> Up to now, I have been able to figure out what to do by reading the
> sources, but it ocured to me that much of my struggles could have been
> mitigated if the relevant types had Show instances.
> 
> I am specifically talking about the types concerning type checking.
> TypecheckedModule and everything below that.
> I am aware that most of the types have an Outputable instance, but
> there are two problems with that:
> 
> - 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)
> - These instances are not intended to show the internal structure of a
>   value, but rather a 'human readable' representation of a value.
> 
> My questions for you:
> 
> - Is there a reason that there are no derived 'Show' instances for most
>   types?
> - Would you accept a diff that adds these?
> 
> Thank you for your time.
> 
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Re: Why are there no Show instances for internal types

[Richard Eisenberg]

My take is that we don't have these because they would slow down compilation 
times and add bloat. But enough people have asked for them (and, I can think of 
a few times when I would use them myself) that I think they should be added.

It is conceivable that we could make the instances only when DEBUG is on. That 
would, I believe, involve some unsavory CPP, and may not be worth it. What do 
others think?

Richard

> On Mar 18, 2017, at 9:03 AM, Tom Sydney Kerckhove  
> wrote:
> 
> Dear GHC Devs,
> 
> I am trying to use GHC as a library but I'm having a lot of trouble with
> understanding what everything means.
> Up to now, I have been able to figure out what to do by reading the
> sources, but it ocured to me that much of my struggles could have been
> mitigated if the relevant types had Show instances.
> 
> I am specifically talking about the types concerning type checking.
> TypecheckedModule and everything below that.
> I am aware that most of the types have an Outputable instance, but
> there are two problems with that:
> 
> - 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)
> - These instances are not intended to show the internal structure of a
>  value, but rather a 'human readable' representation of a value.
> 
> My questions for you:
> 
> - Is there a reason that there are no derived 'Show' instances for most
>  types?
> - Would you accept a diff that adds these?
> 
> Thank you for your time.
> 
> -- 
> Tom Sydney Kerckhove
> ___
> ghc-devs mailing list
> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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Why are there no Show instances for internal types

[Tom Sydney Kerckhove]

Dear GHC Devs,

I am trying to use GHC as a library but I'm having a lot of trouble with
understanding what everything means.
Up to now, I have been able to figure out what to do by reading the
sources, but it ocured to me that much of my struggles could have been
mitigated if the relevant types had Show instances.

I am specifically talking about the types concerning type checking.
TypecheckedModule and everything below that.
I am aware that most of the types have an Outputable instance, but
there are two problems with that:

- 'Outputting' a value requires DynFlags. (yes, I know about pprTrace)
- These instances are not intended to show the internal structure of a
  value, but rather a 'human readable' representation of a value.

My questions for you:

- Is there a reason that there are no derived 'Show' instances for most
  types?
- Would you accept a diff that adds these?

Thank you for your time.

-- 
Tom Sydney Kerckhove


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