[GHC] #10547: feature request: expanding type synonyms in error messages

[GHC]

#10547: feature request: expanding type synonyms in error messages
-------------------------------------+-------------------------------------
        Reporter:  osa1              |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  typecheck/should_fail/ExpandSynsFail1,2,3,4
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1016,
       Wiki Page:                    |  Phab:D2198
-------------------------------------+-------------------------------------

Comment (by ezyang):

 Under Simon's algorithm, I don't see why you should have an `O(n * m)`
 asymptotic behavior. I guess you are trying to handle this case?

 {{{
 type T = Int
 type S = T
 type S1 = S
 type S2 = S

 -- error message comparing S1 and S2
 -- AKA S and S (not Int and Int)
 }}}

 Then the problem resolves to this: given two (singly) linked lists which
 share a common tail, determine the head of the tail. This is a popular
 coding interview question and you can do better than `O(n * m)`; e.g. by
 method 3 here http://www.geeksforgeeks.org/write-a-function-to-get-the-
 intersection-point-of-two-linked-lists/

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10547#comment:18>
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