Opened 3 years ago

Closed 3 years ago

Last modified 3 years ago

#8649 closed bug (fixed)

Disambiguate Repeated Identifiers for data types in error messages

Reported by: agibiansky Owned by:
Priority: normal Milestone:
Component: Compiler Version: 7.6.3
Keywords: Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case: ghci/scripts/T8649
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


If, in an interactive context, you redefine a type, but then try to use functions for the previous definition, it will (as expected) give you an error message. The following code would trigger this in an interactive context:

data X = Y Int
f (Y i) = i
data X = Y Int | Z String
f (Y 3)

However the error message is not very good:

Couldn't match expected type `:Interactive.X'
            with actual type `:Interactive.X'

This is very uninformative to the user if they do not understand what's going on.

The following GHC API code can replicate this:

  runDeclsWithLocation "Interactive.hs" 1 "data X = Y Int"
  runDeclsWithLocation "Asdf.hs" 2 "f (Y i) = i"
  runDeclsWithLocation "Bloop.hs" 3 "data X = Y Int | Z String"
  runStmtWithLocation  "Test.hs" 4 "f (Y 3)" RunToCompletion

Note that varying the location given to runStmt and runDecls (the string as well as the line number) seems to have no effect upon anything. I started out with just "<interactive>" but varied it to check - no effect on anything as far as I can tell.

How can I make this error message more informative? I would ideally like something like

Couldn't match expected type `:Interactive1.X'
            with actual type `:Interactive5.X'

or really anything that caused these types to be visibly different.

Attachments (1)

Test.hs (524 bytes) - added by agibiansky 3 years ago.
Test program demonstrating error

Download all attachments as: .zip

Change History (6)

Changed 3 years ago by agibiansky

Attachment: Test.hs added

Test program demonstrating error

comment:1 Changed 3 years ago by Simon Peyton Jones <simonpj@…>

In 73c08ab10e4077e18e459a1325996bff110360c3/ghc:

Re-work the naming story for the GHCi prompt (Trac #8649)

The basic idea here is simple, and described in Note [The interactive package]
in HscTypes, which starts thus:

    Note [The interactive package]
    Type and class declarations at the command prompt are treated as if
    they were defined in modules
    with each bunch of declarations using a new module, all sharing a
    common package 'interactive' (see Module.interactivePackageId, and

    This scheme deals well with shadowing.  For example:

       ghci> data T = A
       ghci> data T = B
       ghci> :i A
       data Ghci1.T = A  -- Defined at <interactive>:2:10

    Here we must display info about constructor A, but its type T has been
    shadowed by the second declaration.  But it has a respectable
    qualified name (Ghci1.T), and its source location says where it was

    So the main invariant continues to hold, that in any session an original
    name M.T only refers to oe unique thing.  (In a previous iteration both
    the T's above were called :Interactive.T, albeit with different uniques,
    which gave rise to all sorts of trouble.)

This scheme deals nicely with the original problem.  It allows us to
eliminate a couple of grotseque hacks
  - Note [Outputable Orig RdrName] in HscTypes
  - Note [interactive name cache] in IfaceEnv
(both these comments have gone, because the hacks they describe are no
longer necessary). I was also able to simplify Outputable.QueryQualifyName,
so that it takes a Module/OccName as args rather than a Name.

However, matters are never simple, and this change took me an
unreasonably long time to get right.  There are some details in
Note [The interactive package] in HscTypes.

comment:2 Changed 3 years ago by Simon Peyton Jones <simonpj@…>

In 5e8e8e6e62c5826133c053cd78bb35e9b4aaa05b/testsuite:

Changes in error messages when fixing Trac #8649

Mostly improvements, happily

comment:3 Changed 3 years ago by Simon Peyton Jones <simonpj@…>

comment:4 Changed 3 years ago by simonpj

Resolution: fixed
Status: newclosed
Test Case: ghci/scripts/T8649

comment:5 Changed 3 years ago by Simon Peyton Jones <simonpj@…>

In 28e8d878b63d06824001ac3a631254679e0f1960/ghc:

Simplify handling of the interactive package; fixes Trac #8831

This patch is really a fix to the big commint
   Re-work the naming story for the GHCi prompt (Trac #8649)
which introduced the 'interactive' package
See Note [The interactive package] in HscTypes

The original commit set both
  (a) The tcg_mod field of TcGblEnv to 'interactive:Ghci4' (say)
  (b) The thisPackage field of DynFlags to 'interactive'

But the second step interacts badly with linking.  :loaded modules are
in the package set by 'thisPackage' (usually 'main'); if you change
that, then we try to link package 'main', but can't find it, and
that is what happened in #8831.

The fix was simple: do (a) but not (b).

I changed Note [The interactive package] in HscTypes to describe this.
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