resolving Show instance

[Simon Peyton-Jones]

path gets the inferred type
	path :: forall a. [a]

This polymorphic type is instantiated once when it is passed to
'subterm' and again, quite separately, when it is passed to 'shows'.

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org
[mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Serge D. Mechveliani
| Sent: 30 June 2005 11:58
| To: glasgow-haskell-users at haskell.org
| Subject: resolving Show instance
| 
| Who knows, please, why does there happen, now and then, the effects
| of the following kind?
| 
| The  ghc-6-4-branch-June-15-2005  compiler cannot resolve the  Show
| instances in the following program
| (using overlapping and `undecidable' instances):
| 
| -------------------------------------------
| import Dumatel
| main = putStr (shows path $  shows (subterm path t) "\n")
|   where
|   th = bool defaultVariableWeight defaultInitialOpId weightDecls lpo
|                                                     where
|                                                     weightDecls = []
| 
|   t = parseSingleOrBreak th "bool"  "x & (not x)"    :: Term
| 
|   path = []   -- :: SubtermPath
| -------------------------------------------
| 
| 
| It cannot choose the Show instance for  path = [].
| But
| 1) the above `putStr' line contains a subexpression
|                                         shows (subterm path t) "\n",
| 
| 2) the imported modules contain the declarations
| 
|    subterm :: SubtermPath -> Term -> Term
|    ...
|    type SubtermPath = [Integer]
| 
| Hence, it should derive the type for  path :  [Integer].
| 
| Also setting            path = [1]
| or                      path = []  :: SubtermPath
| instead of the line
|                         path = []
| satisfies the compiler.
| 
| This is not a problem for me to guess to add  `:: SubtermPath'.
| This is a question of curiosity.
| The compiler sees to which function `path' is fed to, and sees the
| argument types of this function. Hence, why does not it find the
| type for `path' ?
| 
| I am sure that much simpler example is possible
| (the imported universe is complex), maybe even without overlaps.
| But maybe, people can explain the effect right for this example as
| it is?
| The report is
| 
| ------------------------------------------
|   Overlapping instances for Show [a]
|       arising from use of `shows' at GHCBug.hs:5:15-19
|     Matching instances:
|       Imported from GHC.Show: instance (Show a) => Show [a]
|       Imported from Prelude3: instance Show [Equation]
|       Imported from Prelude3: instance Show [Term]
|     (The choice depends on the instantiation of `a'
|      Use -fallow-incoherent-instances to use the first choice above)
|     In the first argument of `($)', namely `shows path'
|     In the first argument of `putStr', namely
|         `((shows path) $ (shows (subterm path t) "\n"))'
|     In the definition of `main':
|       main = putStr ((shows path) $ (shows (subterm path t) "\n"))
|           where
|           path = []
|           th = bool defaultVariableWeight defaultInitialOpId
weightDecls lpo
|                  where
|                  weightDecls = []
|                  t = parseSingleOrBreak th "bool" "x & (not x)" ::
Term
| --------------------------------------------------------
| 
| 
| Thank you in advance for explanations.
| 
| -----------------
| Serge Mechveliani
| mechvel at botik.ru
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