expandTypeSynonyms panics after kind equality patch

[Simon Peyton Jones]

Richard

I'm confused too.

The 'go' function in 'expandTypeSynonyms' didn't use to take a substitution, but now it does.  I think you intend that

  go subst ty = expand (substTy subst ty)

where 'go' is the new 'go' and 'expand' is 'expandTypeSynonyms'. Correct?  So you are avoiding repeated traversal by carrying a substitution in with you.

It would be good to write down this invariant.

OK so suppose we have

  new_go subst (T ty1 ty2)

where type T a = rhs.  (Notice T has arity 1 but is applied to two args.)

Then we this should be equivalent to

  expand (T (subst(ty1)) (subst(ty2))
which is
  expand (rhs[a -> subst ty1]  subst(ty2))


So instead of
    go subst (TyConApp tc tys)
      | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys
      = let subst' = unionTCvSubst subst (mkTopTCvSubst tenv) in
        go subst' (mkAppTys rhs tys')

I'd expect to see
    go subst (TyConApp tc tys)
      | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc 
                                     (map (go subst) tys)
      = mkAppTys (go (mkTopTCvSubst tenv) rhs) tys'

Notice that I'm applying 'go subst' to 'tys' before giving the args to 'expandSynTyCon_maybe'.

Does that look right?  What you have looks entirely inexplicable.

Simon


|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ömer
|  Sinan Agacan
|  Sent: 06 January 2016 16:34
|  To: ghc-devs <ghc-devs at haskell.org>
|  Subject: expandTypeSynonyms panics after kind equality patch
|  
|  My branch panicking during stage 2 build and when I tried to debug I
|  realized the panicking function is `unionTCvSubst`, when called by
|  `expandTypeSynonyms`.
|  In my branch I'm doing some type-based transformations and I'm using
|  `expandTypeSynonyms` on type of identifiers for that.
|  
|  According to git blame logs, the function `unionTCvSubst` was added
|  with kind equality patch. The patch made this change in
|  `expandTypeSynonyms`:
|  
|      -      = go (mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys')
|      +      = let subst' = unionTCvSubst subst (mkTopTCvSubst tenv) in
|      +        go subst' (mkAppTys rhs tys')
|  
|  Which is what's causing the panic. The exact place it's panicking
|  during the
|  stage2 build is when I call `expandTypeSynonyms` on type `TvSubstEnv`
|  (i.e. in my transformation I get type of an id and it turns out to be
|  TvSubstEnv, when I call `expandTypeSynonyms` on this type it panics)
|  
|  I figured this much but I don't understand type system details, so,
|  does anyone here have any ideas what's going wrong here? Richard?
|  
|  Note that this panic happens even though I don't do any changes in
|  types - I'm just looking at the types for some transformations but no
|  changes on the types are done.
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