I have been under the impression that we don't even want to print those. On the contrary, inside GHC I /do/ want to print them. Otherwise how can I see what the renamer has done? Simon From: Shayan Najd [mailto:sh.najd@gmail.com] Sent: 28 July 2017 12:20 To: Simon Peyton Jones Cc: ghc-devs@haskell.org; Alan & Kim Zimmerman Subject: Re: Operating on HsSyn Before all this, we may need to discuss a bit about the intended semantics of `Outputable`: does it need to print `PostRn`, or `PostTc` fields; or `Out` suffixed constructors? If not, then we only need to write a set of instances for the base growable AST, once and for all. Such instances will be polymorphic on the extension descriptor `p`, and do not need to mention the constraints like `(PostRn p (IdP p)`, since these are just extensions and not part of the base growable AST. Or, am I missing something about the intended semantics of `Outputable`? You write So today we never print these annotations, to avoid bloating the instance contexts, which can be painful. I have been under the impression that we don't even want to print those. Of course, there are scenarios (like `Show` instances) where we do want to write compositional / generic functions that take into account the extensions. Here is my abstract overview of the scenario, that may help the discussion. Consider data types `A`, `B`, and `C` (say, one AST datatype per compiler phase) that are defined as extensions to a base datatype `T`:

A = T XA B = T XB C = T XC

where `X*`s are extension descriptors. Now, say we want to a define functions `f_A`, `f_B`, and `f_C` over `A`, `B`, and `C`. We have two main alternatives: (a) either we write these (manually or using the deriving mechanism) separately (b) or we write a generic / parametric function `g` over `T`, and reuse that to define `f_*`s Of course, (b) is preferable in theory , but not always possible or preferable in practice. In which case, we can always resort to (a). The more varying are the definitions of `f_A`, `f_B`, and `f_C` the more parametric should `g` get, as this is the case for any generic function. With a correct design, I believe, these are all independent of Trees that Grow story itself: we are now not only trying to reuse data types, and functions agnostic towards extensions (pretty printers in my view of their semantics), but also reuse functions with parametric / varying behaviour with respect to extensions. /Shayan On Fri, Jul 28, 2017 at 10:18 AM, Simon Peyton Jones mailto:simonpj@microsoft.com> wrote: Devs, Shayan is working away on “Trees that grow”… do keep it on your radar: To: ghc-devs Sent: 25 May 2017 23:49 Do take a look at this: • We propose to re-engineer HsSyn itself. This will touch a lot of code. • But it’s very neat, and will bring big long-term advantages • And we can do it a bit at a time The wiki page https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow has the details. It’s entirely an internal change, not a change to GHC’s specification, so it’s independent of the GHC proposals process. But I’d value the opinion of other GHC devs Meanwhile I have a question. When pretty-printing HsSyn we often have a situation like this: data Pass = Parsed | Renamed | Typechecked data HsExpr (p :: Pass) = HsVar (IdP p) | .... type famliy IdP p where IdP Parsed = RdrName IdP Renamed = Name IdP Typechecked = Id instance (Outputable (IdP p)) => Outputable (HsExpr p) where ppr (HsVar v) = ppr v The (ppr v) requires (Outputable (IdP p)), hence the context. Moreover, and more seriously, there are things we just can't pretty-print right now. For example, HsExpr has this data constructor: data HsExpr p = ... | OpApp (LHsExpr p) (LHsExpr p) (PostRn p Fixity) (LHsExpr p) To pretty-print the third argument, we'd need to add instance (Outputable (IdP p), Outputable (PostRn p Fixity)) -- New => Outputable (HsExpr p) where ppr (HsVar v) = ppr v and that gets onerous. So today we never print these annotations, to avoid bloating the instance contexts, which can be painful. It bit me yesterday. We have bitten that bullet for the Data class: look at HsExtension.DataId, which abbreviates the long list of dictionaries: type DataId p = ( Data p , ForallX Data p , Data (NameOrRdrName (IdP p)) , Data (IdP p) , Data (PostRn p (IdP p)) , Data (PostRn p (Located Name)) , Data (PostRn p Bool) , Data (PostRn p Fixity) ,..and nine more... ) Let me note in passing that [wiki:QuantifiedContextshttps://ghc.haskell.org/trac/ghc/wiki/QuantifiedContexts] would make this somewhat shorter type DataId p = ( Data p , ForallX Data p , Data (NameOrRdrName (IdP p)) , Data (IdP p) , forall t. Data t => Data (PostRn p t)) But we still need one item in this list for each type function, and I am worried about how this scales to the [wiki:ImplementingTreesThatGrowhttps://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow] story, when we have a type function for each data constructor -- and there are a lot of data constructors. So I have four questions 1. I think we should probably use a superclass instead of a type synonym class (Data p, ForallX Data p, ....) => DataId p where {} Why? Only one argument to pass, and to pass on to successive calls. I see no downside. 1. Shall we treat Outputable like Data? (I.e. make an abbreviation for a long list of Outputable instances and use it everywhere) 2. I thought of another way to do it (pass a token); see below 1. Are there any other ways? Token passing idea. Perhaps instead of passing lots of functions, we pass a singleton token that encodes the pass, like this: instance (PassC p) => Outputable (HsExpr p) where ppr (HsVar v) = case getPass :: IsPass p of IsParsed -> ppr v IsRenamed -> ppr v IsTypechecked -> ppr v The three ppr's are at different types, of course; that's the point. The infrastructure is something like class PassC p where getPass :: IsPass p data IsPass p where IsParsed :: IsPass Parsed IsRenamed :: IsParsed Renamed IsTypechecked :: IsParsed Typechecked instance PassC Parsed where getPass = IsParsed ...etc... Now we could sweep away all those OutputableX classes, replacing them with dynamic tests on the singletons IsParsed etc. This would have advantages: - Probably faster: there's a dynamic test, but many fewer dictionary arguments and higher-order function dispatch - Only one dictionary to pass; programming is easier. The big downside is that it's not extensible: it works only because we know the three cases. But the "Trees that Grow" story really doesn't scale well to pretty-printing: so maybe we should just give up on that?

by (parser . prettyPrint . parser) = id I meant (prettyPrint . parser . prettyPrint) = id for a valid input. On Fri, Jul 28, 2017 at 4:32 PM, Shayan Najd wrote:

On the contrary, inside GHC I /do/ want to print them. Otherwise how can I

...

see what the renamer has done?

Right. So if I understand correctly, with this semantics, `Outputable` is somewhere between pretty printing as often used in program manipulation libraries (like Haskell-Src-Exts (HSE)) which is closer to syntax, and `Show` which is closer to Haskell representation. (There are also "exact printers" (as in HSE) that are even closer to syntax in some sense.) Often, pretty printers generate only grammatically valid terms, not the ones polluted with extra annotations (hence grammatically invalid), e.g., what is the grammatically valid form of `OpApp` printed via `Outputable` that includes the fixity annotation.

If I recall correctly, we have briefly studied these in the past summer, we came up with some roundtrip correctness criteria, like the following (bar error handling; assume valid input):

(parser . prettyPrint . parser) = id

[paging in Jacques]

The reason I am trying to flesh out the semantics is the /big/ gains on code reuse later on in the process: one does not need to define a separate pretty printing library for Haskell syntax, and can reuse the well-tested and well-maintained one in GHC.

Reformulating part of your concern, based on my understanding (if I may), the questions is: what is the proper design of an "outputer" (debug-printer?) where /annotated/ terms can be pretty-printed including any printable (pretty?showable?) annotations. In particular, we may want to take advantage of extensibility of data types for this. Am I far off?

Note: with proper design, an extensible debug-printer can still subsume corresponding pretty-printers.

On Fri, Jul 28, 2017 at 2:36 PM, Simon Peyton Jones

...

wrote:

...

I have been under the impression that we don't even want to print those.

On the contrary, inside GHC I /do/ want to print them. Otherwise how can I see what the renamer has done?

Simon

*From:* Shayan Najd [mailto:sh.najd@gmail.com] *Sent:* 28 July 2017 12:20 *To:* Simon Peyton Jones *Cc:* ghc-devs@haskell.org; Alan & Kim Zimmerman *Subject:* Re: Operating on HsSyn

Before all this, we may need to discuss a bit about the intended semantics of

`Outputable`: does it need to print `PostRn`, or `PostTc` fields; or `Out`

suffixed constructors? If not, then we only need to write a set of instances

for the base growable AST, once and for all. Such instances will be polymorphic

on the extension descriptor `p`, and do not need to mention the constraints like

`(PostRn p (IdP p)`, since these are just extensions and not part of the base

growable AST. Or, am I missing something about the intended semantics of

`Outputable`?

You write

So today we never print these annotations, to avoid bloating the instance contexts, which can be painful.

I have been under the impression that we don't even want to print those.

Of course, there are scenarios (like `Show` instances) where we do want to write

compositional / generic functions that take into account the extensions.

Here is my abstract overview of the scenario, that may help the discussion.

Consider data types `A`, `B`, and `C` (say, one AST datatype per compiler phase) that

are defined as extensions to a base datatype `T`:

...

A = T XA

...

B = T XB

...

C = T XC

where `X*`s are extension descriptors.

Now, say we want to a define functions `f_A`, `f_B`, and `f_C` over `A`, `B`, and `C`.

We have two main alternatives:

(a) either we write these (manually or using the deriving mechanism) separately

(b) or we write a generic / parametric function `g` over `T`, and reuse that to define `f_*`s

Of course, (b) is preferable in theory , but not always possible or preferable in practice.

In which case, we can always resort to (a).

The more varying are the definitions of `f_A`, `f_B`, and `f_C` the more parametric should

`g` get, as this is the case for any generic function.

With a correct design, I believe, these are all independent of Trees that Grow story itself:

we are now not only trying to reuse data types, and functions agnostic towards extensions

(pretty printers in my view of their semantics), but also reuse functions with parametric /

varying behaviour with respect to extensions.

/Shayan

On Fri, Jul 28, 2017 at 10:18 AM, Simon Peyton Jones < simonpj@microsoft.com> wrote:

Devs,

Shayan is working away on “Trees that grow”… do keep it on your radar:

*To:* ghc-devs *Sent:* 25 May 2017 23:49

Do take a look at this:

· We propose to re-engineer HsSyn itself. This will touch a *lot* of code.

· But it’s very neat, and will bring big long-term advantages

· And we can do it a bit at a time

The wiki page https://ghc.haskell.org/trac/g hc/wiki/ImplementingTreesThatGrow has the details. It’s entirely an internal change, not a change to GHC’s specification, so it’s independent of the GHC proposals process. But I’d value the opinion of other GHC devs

Meanwhile I have a question. When pretty-printing HsSyn we often have a situation like this:

data Pass = Parsed | Renamed | Typechecked

data HsExpr (p :: Pass) = HsVar (IdP p) | ....

type famliy IdP p where

IdP Parsed = RdrName

IdP Renamed = Name

IdP Typechecked = Id

instance (Outputable (IdP p)) => Outputable (HsExpr p) where

ppr (HsVar v) = ppr v

The (ppr v) requires (Outputable (IdP p)), hence the context.

Moreover, and more seriously, there are things we just can't pretty-print

right now. For example, HsExpr has this data constructor:

data HsExpr p = ...

| OpApp (LHsExpr p)

(LHsExpr p)

(PostRn p Fixity)

(LHsExpr p)

To pretty-print the third argument, we'd need to add

instance (Outputable (IdP p),

Outputable (PostRn p Fixity)) -- New

=> Outputable (HsExpr p) where

ppr (HsVar v) = ppr v

and that gets onerous. *So today we never print these annotations*, to avoid bloating the instance contexts, which can be painful. It bit me yesterday.

We have bitten that bullet for the Data class: look at HsExtension.DataId, which abbreviates the long list of dictionaries:

type DataId p =

( Data p

, ForallX Data p

, Data (NameOrRdrName (IdP p))

, Data (IdP p)

, Data (PostRn p (IdP p))

, Data (PostRn p (Located Name))

, Data (PostRn p Bool)

, Data (PostRn p Fixity)

,..and nine more... )

Let me note in passing that [wiki:QuantifiedContexts https://ghc.haskell.org/trac/ghc/wiki/QuantifiedContexts] would make this somewhat shorter

type DataId p =

( Data p

, ForallX Data p

, Data (NameOrRdrName (IdP p))

, Data (IdP p)

, forall t. Data t => Data (PostRn p t))

But we still need one item in this list for each type function,

and I am worried about how this scales to the

[wiki:ImplementingTreesThatGrow https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow] story, when we have a type

function for each data constructor -- and there are a *lot* of data

constructors.

*So I have four questions*

1. I think we should probably use a superclass instead of a type synonym

class (Data p, ForallX Data p, ....) => DataId p where {}

Why? Only one argument to pass, and to pass on to successive calls. I see no downside.

1. Shall we treat Outputable like Data? (I.e. make an abbreviation for a long list of Outputable instances and use it everywhere) 2. I thought of another way to do it (pass a token); see below

1. Are there any other ways?

*Token passing idea.*

Perhaps instead of passing lots of functions, we pass a singleton token

that encodes the pass, like this:

instance (PassC p) => Outputable (HsExpr p) where

ppr (HsVar v) = case getPass :: IsPass p of

IsParsed -> ppr v

IsRenamed -> ppr v

IsTypechecked -> ppr v

The three ppr's are at different types, of course; that's the point.

The infrastructure is something like

class PassC p where

getPass :: IsPass p

data IsPass p where

IsParsed :: IsPass Parsed

IsRenamed :: IsParsed Renamed

IsTypechecked :: IsParsed Typechecked

instance PassC Parsed where getPass = IsParsed

...etc...

Now we could sweep away all those OutputableX classes,

replacing them with dynamic tests on the singletons IsParsed etc.

This would have advantages:

- Probably faster: there's a dynamic test, but many fewer dictionary

arguments and higher-order function dispatch

- Only one dictionary to pass; programming is easier.

The big downside is that it's not extensible: it works only because

we know the three cases. But the "Trees that Grow" story really doesn't

scale well to pretty-printing: so maybe we should just give up on that?

by

(parser . prettyPrint . parser) = id

I meant

(prettyPrint . parser . prettyPrint) = id

for a valid input.

Simplifying, (parser ∷ String → something), and (prettyPrint ∷ something → String). Therefore, (parser . prettyPrint . parser ∷ String → something) and (prettyPrint . parser . prettyPrint ∷ something → String). Therefore, both criteria could only apply for (something ~ String). But as pretty printing adds quotation marks, not even that is true. There are four formulations that might be applicable: 1. parser . prettyPrint ≍ id 2. prettyPrint . parser ≍ id -- ∷ String → String, useless here 3. prettyPrint . parser . prettyPrint ≍ prettyPrint 4. parser . prettyPrint . parser ≍ parser 5. Well, you could go beyond to (prettyPrint . parser . prettyPrint. parser ≍prettyPrint. parser) etc… I don't think 1 (or 2) follow from one of the last two. But 1 does imply them. So it is a stronger criterion than both, and therefore probably not the one to choose. Assuming the parser is internally consistent, 3 just says something about the internal consistency of the pretty printer, while 4 says something about the relationship of the pretty printer to the parser. Thus 4 looks like the best candidate for a criterion. Possibly with 3 as a secondary target. Cheers, MarLinn

MarLinn, Thanks for correcting me, and spelling this out. I did mean what Alan mentioned: "re-parsing a pretty printed parse tree gives you back a parse tree identical to the original (ignoring SrcSpans)". As I recall, we had to go a bit further to give 'Something' some more structure to take into account things like "(ignoring SrcSpans)" (e.g., to define exact-printers, etc). Provided I have failed twice to properly recall the invariant, I refrain from trying to recall the rest tonight :) Not diverging from my point above, as far as I understand, an ideal `Outputable` machinery is going to be a bit different from the traditional pretty printers. I believe with a proper design we can even reuse `Outputable` machinery and provide it as a pretty printer for Haskell terms. It resembles the scenario in Section 3.7 compared to Section 3.6 of Trees that Grow [1]. Having said all these, we ARE diverging from the original thread, and Simon's questions. How about taking printer-design related discussion to the following wiki page (and/or a new ghc-dev thread if needed): https://ghc.haskell.org/trac/ghc/wiki/HaskellSyntaxPrinters Cheers, Shayan [1] http://www.jucs.org/jucs_23_1/trees_that_grow/jucs_23_01_0042_0062_najd.pdf On Fri, Jul 28, 2017 at 8:43 PM, Alan & Kim Zimmerman wrote:

I agree. 4 is the current GHC invariant.

i.e., re-parsing a pretty printed parse tree gives you back a parse tree identical to the original (ignoring SrcSpans)

Alan

On 28 July 2017 at 20:34, MarLinn wrote:

...

by

(parser . prettyPrint . parser) = id

I meant

(prettyPrint . parser . prettyPrint) = id

for a valid input.

Simplifying, (parser ∷ String → something), and (prettyPrint ∷ something → String).

Therefore, (parser . prettyPrint . parser ∷ String → something) and (prettyPrint . parser . prettyPrint ∷ something → String).

Therefore, both criteria could only apply for (something ~ String). But as pretty printing adds quotation marks, not even that is true.

There are four formulations that might be applicable:

1.

parser . prettyPrint ≍ id 2.

prettyPrint . parser ≍ id -- ∷ String → String, useless here 3.

prettyPrint . parser . prettyPrint ≍ prettyPrint 4.

parser . prettyPrint . parser ≍ parser 5. Well, you could go beyond to (prettyPrint . parser . prettyPrint . parser ≍ prettyPrint . parser) etc…

I don't think 1 (or 2) follow from one of the last two. But 1 does imply them. So it is a stronger criterion than both, and therefore probably not the one to choose. Assuming the parser is internally consistent, 3 just says something about the internal consistency of the pretty printer, while 4 says something about the relationship of the pretty printer to the parser. Thus 4 looks like the best candidate for a criterion. Possibly with 3 as a secondary target.

Cheers, MarLinn

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Am I far off? I don’t think you are far off. All I’m after is the debug-printer use-case: I want to be able to see the contents of the tree, including the PostRn annotations. I don’t care about round-tripping it. Simon From: Shayan Najd [mailto:sh.najd@gmail.com] Sent: 28 July 2017 15:32 To: Simon Peyton Jones Cc: ghc-devs@haskell.org; Alan & Kim Zimmerman ; Jacques Carette Subject: Re: Operating on HsSyn On the contrary, inside GHC I /do/ want to print them. Otherwise how can I see what the renamer has done? Right. So if I understand correctly, with this semantics, `Outputable` is somewhere between pretty printing as often used in program manipulation libraries (like Haskell-Src-Exts (HSE)) which is closer to syntax, and `Show` which is closer to Haskell representation. (There are also "exact printers" (as in HSE) that are even closer to syntax in some sense.) Often, pretty printers generate only grammatically valid terms, not the ones polluted with extra annotations (hence grammatically invalid), e.g., what is the grammatically valid form of `OpApp` printed via `Outputable` that includes the fixity annotation. If I recall correctly, we have briefly studied these in the past summer, we came up with some roundtrip correctness criteria, like the following (bar error handling; assume valid input): (parser . prettyPrint . parser) = id [paging in Jacques] The reason I am trying to flesh out the semantics is the /big/ gains on code reuse later on in the process: one does not need to define a separate pretty printing library for Haskell syntax, and can reuse the well-tested and well-maintained one in GHC. Reformulating part of your concern, based on my understanding (if I may), the questions is: what is the proper design of an "outputer" (debug-printer?) where /annotated/ terms can be pretty-printed including any printable (pretty?showable?) annotations. In particular, we may want to take advantage of extensibility of data types for this. Am I far off? Note: with proper design, an extensible debug-printer can still subsume corresponding pretty-printers. On Fri, Jul 28, 2017 at 2:36 PM, Simon Peyton Jones mailto:simonpj@microsoft.com> wrote: I have been under the impression that we don't even want to print those. On the contrary, inside GHC I /do/ want to print them. Otherwise how can I see what the renamer has done? Simon From: Shayan Najd [mailto:sh.najd@gmail.commailto:sh.najd@gmail.com] Sent: 28 July 2017 12:20 To: Simon Peyton Jones mailto:simonpj@microsoft.com> Cc: ghc-devs@haskell.orgmailto:ghc-devs@haskell.org; Alan & Kim Zimmerman mailto:alan.zimm@gmail.com> Subject: Re: Operating on HsSyn Before all this, we may need to discuss a bit about the intended semantics of `Outputable`: does it need to print `PostRn`, or `PostTc` fields; or `Out` suffixed constructors? If not, then we only need to write a set of instances for the base growable AST, once and for all. Such instances will be polymorphic on the extension descriptor `p`, and do not need to mention the constraints like `(PostRn p (IdP p)`, since these are just extensions and not part of the base growable AST. Or, am I missing something about the intended semantics of `Outputable`? You write So today we never print these annotations, to avoid bloating the instance contexts, which can be painful. I have been under the impression that we don't even want to print those. Of course, there are scenarios (like `Show` instances) where we do want to write compositional / generic functions that take into account the extensions. Here is my abstract overview of the scenario, that may help the discussion. Consider data types `A`, `B`, and `C` (say, one AST datatype per compiler phase) that are defined as extensions to a base datatype `T`:

A = T XA B = T XB C = T XC

where `X*`s are extension descriptors. Now, say we want to a define functions `f_A`, `f_B`, and `f_C` over `A`, `B`, and `C`. We have two main alternatives: (a) either we write these (manually or using the deriving mechanism) separately (b) or we write a generic / parametric function `g` over `T`, and reuse that to define `f_*`s Of course, (b) is preferable in theory , but not always possible or preferable in practice. In which case, we can always resort to (a). The more varying are the definitions of `f_A`, `f_B`, and `f_C` the more parametric should `g` get, as this is the case for any generic function. With a correct design, I believe, these are all independent of Trees that Grow story itself: we are now not only trying to reuse data types, and functions agnostic towards extensions (pretty printers in my view of their semantics), but also reuse functions with parametric / varying behaviour with respect to extensions. /Shayan On Fri, Jul 28, 2017 at 10:18 AM, Simon Peyton Jones mailto:simonpj@microsoft.com> wrote: Devs, Shayan is working away on “Trees that grow”… do keep it on your radar: To: ghc-devs Sent: 25 May 2017 23:49 Do take a look at this: • We propose to re-engineer HsSyn itself. This will touch a lot of code. • But it’s very neat, and will bring big long-term advantages • And we can do it a bit at a time The wiki page https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow has the details. It’s entirely an internal change, not a change to GHC’s specification, so it’s independent of the GHC proposals process. But I’d value the opinion of other GHC devs Meanwhile I have a question. When pretty-printing HsSyn we often have a situation like this: data Pass = Parsed | Renamed | Typechecked data HsExpr (p :: Pass) = HsVar (IdP p) | .... type famliy IdP p where IdP Parsed = RdrName IdP Renamed = Name IdP Typechecked = Id instance (Outputable (IdP p)) => Outputable (HsExpr p) where ppr (HsVar v) = ppr v The (ppr v) requires (Outputable (IdP p)), hence the context. Moreover, and more seriously, there are things we just can't pretty-print right now. For example, HsExpr has this data constructor: data HsExpr p = ... | OpApp (LHsExpr p) (LHsExpr p) (PostRn p Fixity) (LHsExpr p) To pretty-print the third argument, we'd need to add instance (Outputable (IdP p), Outputable (PostRn p Fixity)) -- New => Outputable (HsExpr p) where ppr (HsVar v) = ppr v and that gets onerous. So today we never print these annotations, to avoid bloating the instance contexts, which can be painful. It bit me yesterday. We have bitten that bullet for the Data class: look at HsExtension.DataId, which abbreviates the long list of dictionaries: type DataId p = ( Data p , ForallX Data p , Data (NameOrRdrName (IdP p)) , Data (IdP p) , Data (PostRn p (IdP p)) , Data (PostRn p (Located Name)) , Data (PostRn p Bool) , Data (PostRn p Fixity) ,..and nine more... ) Let me note in passing that [wiki:QuantifiedContextshttps://ghc.haskell.org/trac/ghc/wiki/QuantifiedContexts] would make this somewhat shorter type DataId p = ( Data p , ForallX Data p , Data (NameOrRdrName (IdP p)) , Data (IdP p) , forall t. Data t => Data (PostRn p t)) But we still need one item in this list for each type function, and I am worried about how this scales to the [wiki:ImplementingTreesThatGrowhttps://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow] story, when we have a type function for each data constructor -- and there are a lot of data constructors. So I have four questions 1. I think we should probably use a superclass instead of a type synonym class (Data p, ForallX Data p, ....) => DataId p where {} Why? Only one argument to pass, and to pass on to successive calls. I see no downside. 1. Shall we treat Outputable like Data? (I.e. make an abbreviation for a long list of Outputable instances and use it everywhere) 2. I thought of another way to do it (pass a token); see below 1. Are there any other ways? Token passing idea. Perhaps instead of passing lots of functions, we pass a singleton token that encodes the pass, like this: instance (PassC p) => Outputable (HsExpr p) where ppr (HsVar v) = case getPass :: IsPass p of IsParsed -> ppr v IsRenamed -> ppr v IsTypechecked -> ppr v The three ppr's are at different types, of course; that's the point. The infrastructure is something like class PassC p where getPass :: IsPass p data IsPass p where IsParsed :: IsPass Parsed IsRenamed :: IsParsed Renamed IsTypechecked :: IsParsed Typechecked instance PassC Parsed where getPass = IsParsed ...etc... Now we could sweep away all those OutputableX classes, replacing them with dynamic tests on the singletons IsParsed etc. This would have advantages: - Probably faster: there's a dynamic test, but many fewer dictionary arguments and higher-order function dispatch - Only one dictionary to pass; programming is easier. The big downside is that it's not extensible: it works only because we know the three cases. But the "Trees that Grow" story really doesn't scale well to pretty-printing: so maybe we should just give up on that?