Stan 0206 should not fire for type data

[noughtmare]

I have this code:

{-# LANGUAGE TypeData #-}

infixr :->
type data PCFType = PCFType :-> PCFType | Nat

And Stan reports:

 ✲ Name:        Data types with non-strict fields
 ✲ Description: Defining lazy fields in data types can lead to unexpected space leaks
 ✲ Severity:    Performance
 ✲ Category:    #SpaceLeak #Syntax
Possible solutions:
  - Add '!' before the type, e.g. !Int or !(Maybe Bool)
  - Enable the 'StrictData' extension: {-# LANGUAGE StrictData #-}
stan(STAN-0206)

But strictness fields are not allowed in type data declarations.

[tomjaguarpaw]

Thanks, I agree this is a bug. I suspect it will only be fixed if there's a volunteer contribution.

[noughtmare]

I've looked into it a bit and I think the change should be somewhere here:

stan/src/Stan/Analysis/Analyser.hs

Lines 167 to 180 in ddcd461

	matchLazyField node = memptyIfFalse
	-- return empty list if it's not a data type
	(hieMatchPatternAst hie node dataDecl)
	-- get list of all constructors
	$ let constructors = filter
	(\n -> hieMatchPatternAst hie n constructor)
	(nodeChildren node)
	in case constructors of
	-- no constructors = not observations
	[] -> mempty
	-- single constructor
	[c] -> S.concatMap matchField $ extractFields False c
	-- multiple constructors = analyse everything
	cs -> S.concatMap (S.concatMap matchField . extractFields True) cs

But I don't quite know what to change yet.

[0rphee]

I've looked into this, and I think there should be another check, similar to this (hieMatchPatternAst hie node dataDecl), but checking that these are type data declarations and not only data declarations. Kinda something like this:

-- Original
{- | @data@ or @newtype@ declaration.
-}
dataDecl :: PatternAst
dataDecl = PatternAstNode $ one (mkNodeAnnotation "DataDecl" "TyClDecl")

-- new
{- | @type data@ declaration.
-}
typeDataDecl :: PatternAst
typeDataDecl = PatternAstNode $ one (mkNodeAnnotation "MODIFIED" "MODIFIED")

However, this seems not to be possible:

Checking with ghc Main.hs -fwrite-ide-info && hiedb index . && hiedb dump Main.hie, I get this results:

{-# LANGUAGE TypeData #-}

module Main where

type data TypeData = TD1 | TD2

data Data = D1 | D2

main :: IO ()
main = pure ()

Command output

rocessing file 1/1: /Users/roger/dev/haskell/test/Main.hie... skipped                                                                                                                                      
Completed! (0 indexed, 1 skipped in 0.00s + 0.00s gc)
Node@Main.hs:(1,1)-(10,14): Source: From source
                            {(annotations: {(Module, Module)}),  (types: []), 
                             (identifier info: {})}
                            
  Node@Main.hs:1:1: Source: From source
                    {(annotations: {}),  (types: []), 
                     (identifier info: {(name $dApplicative,  Details:  Just 1 {evidence variable bound by a let, depending on: [$fApplicativeIO]
                                                                                with scope: ModuleScope
                                                                                }), 
                                        (name $fApplicativeIO,  Details:  Just 1 {usage of evidence variable})})}
                    
  Node@Main.hs:5:1-30: Source: From source
                       {(annotations: {(DataDecl, TyClDecl)}),  (types: []), 
                        (identifier info: {})}
                       
    Node@Main.hs:5:11-18: Source: From source
                          {(annotations: {}),  (types: []), 
                           (identifier info: {(name TypeData,  Details:  Nothing {declaration of data bound at: Main.hs:5:1-30})})}
                          
    Node@Main.hs:5:22-24: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name TD1,  Details:  Nothing {declaration of constructor bound at: Main.hs:5:22-24})})}
                          
    Node@Main.hs:5:28-30: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name TD2,  Details:  Nothing {declaration of constructor bound at: Main.hs:5:28-30})})}
                          
  Node@Main.hs:7:1-19: Source: From source
                       {(annotations: {(DataDecl, TyClDecl)}),  (types: []), 
                        (identifier info: {})}
                       
    Node@Main.hs:7:6-9: Source: From source
                        {(annotations: {}),  (types: []), 
                         (identifier info: {(name Data,  Details:  Nothing {declaration of data bound at: Main.hs:7:1-19})})}
                        
    Node@Main.hs:7:13-14: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name D1,  Details:  Nothing {declaration of constructor bound at: Main.hs:7:13-14})})}
                          
    Node@Main.hs:7:18-19: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name D2,  Details:  Nothing {declaration of constructor bound at: Main.hs:7:18-19})})}
                          
  Node@Main.hs:9:1-13: Source: From source
                       {(annotations: {(TypeSig, Sig)}),  (types: []), 
                        (identifier info: {})}
                       
    Node@Main.hs:9:1-4: Source: From source
                        {(annotations: {}),  (types: []), 
                         (identifier info: {(name main,  Details:  Nothing {bound in a type signature declaration})})}
                        
    Node@Main.hs:9:9-13: Source: From source
                         {(annotations: {(HsAppTy, HsType), (HsSig, HsSigType)}), 
                          (types: []),  (identifier info: {})}
                         
      Node@Main.hs:9:9-10: Source: From source
                           {(annotations: {(HsTyVar, HsType)}),  (types: []), 
                            (identifier info: {(name IO,  Details:  Nothing {usage})})}
                           
      Node@Main.hs:9:12-13: Source: From source
                            {(annotations: {(HsTupleTy, HsType)}),  (types: []), 
                             (identifier info: {})}
                            
  Node@Main.hs:10:1-14: Source: From source
                        {(annotations: {(FunBind, HsBindLR), (Match, Match),
                                        (XHsBindsLR, HsBindLR)}), 
                         (types: [3]),  (identifier info: {})}
                        
    Node@Main.hs:10:1-4: Source: From source
                         {(annotations: {(HsVar, HsExpr)}),  (types: [3]), 
                          (identifier info: {(name main,  Details:  Just 3 {usage,
                                                                            LHS of a match group,
                                                                            regular value bound with scope: ModuleScope bound at: Main.hs:10:1-14})})}
                         
    Node@Main.hs:10:6-14: Source: From source
                          {(annotations: {(GRHS, GRHS)}),  (types: []), 
                           (identifier info: {})}
                          
      Node@Main.hs:10:8-14: Source: From source
                            {(annotations: {(HsApp, HsExpr)}),  (types: []), 
                             (identifier info: {})}
                            
        Node@Main.hs:10:8-11: Source: From source
                              {(annotations: {(HsVar, HsExpr), (XExpr, HsExpr)}), 
                               (types: [5, 10, 18]), 
                               (identifier info: {(name pure,  Details:  Just 18 {usage}), 
                                                  (name $dApplicative,  Details:  Just 1 {usage of evidence variable})})}
                              
        Node@Main.hs:10:13-14: Source: From source
                               {(annotations: {(HsVar, HsExpr), (XExpr, HsExpr)}),  (types: [2]), 
                                (identifier info: {(name (),  Details:  Nothing {usage})})}
                               

Crucially, there's no difference between the type data and data...

 
  Node@Main.hs:5:1-30: Source: From source
                       {(annotations: {(DataDecl, TyClDecl)}),  (types: []), 
                        (identifier info: {})}
                       
    Node@Main.hs:5:11-18: Source: From source
                          {(annotations: {}),  (types: []), 
                           (identifier info: {(name TypeData,  Details:  Nothing {declaration of data bound at: Main.hs:5:1-30})})}
                          
    Node@Main.hs:5:22-24: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name TD1,  Details:  Nothing {declaration of constructor bound at: Main.hs:5:22-24})})}
                          
    Node@Main.hs:5:28-30: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name TD2,  Details:  Nothing {declaration of constructor bound at: Main.hs:5:28-30})})}
                          
  Node@Main.hs:7:1-19: Source: From source
                       {(annotations: {(DataDecl, TyClDecl)}),  (types: []), 
                        (identifier info: {})}
                       
    Node@Main.hs:7:6-9: Source: From source
                        {(annotations: {}),  (types: []), 
                         (identifier info: {(name Data,  Details:  Nothing {declaration of data bound at: Main.hs:7:1-19})})}
                        
    Node@Main.hs:7:13-14: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name D1,  Details:  Nothing {declaration of constructor bound at: Main.hs:7:13-14})})}
                          
    Node@Main.hs:7:18-19: Source: From source
                          {(annotations: {(ConDeclH98, ConDecl)}),  (types: []), 
                           (identifier info: {(name D2,  Details:  Nothing {declaration of constructor bound at: Main.hs:7:18-19})})}
                          

(I checked this on 9.8.4, 9.10.1 and 9.12.2)