[GHC] #10837: Constant-time indexing of closed type family axioms

[GHC]

#10837: Constant-time indexing of closed type family axioms
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              Reporter:  jstolarek   |             Owner:
                  Type:  task        |            Status:  new
              Priority:  low         |         Milestone:
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Right now `CoAxiom` stores a list of `CoAxBranch`es in a `BranchList`. But
 it would be useful to actually have a constant-time indexing into axioms.
 One code fragment when I really wanted to have this feature is in
 `TcValidity.checkValidCoAxiom`:

 {{{#!hs
 -- Replace n-th element in the list. Assumes 0-based indexing.
 replace_br :: [CoAxBranch] -> Int -> CoAxBranch -> [CoAxBranch]
 replace_br brs n br = take n brs ++ [br] ++ drop (n+1) brs
 }}}

 I believe this code could be rewritten in a much more efficient way using
 constant-time indexing.

 The new data structure should most likely have a type that indicates
 whether the axiom is branched or not.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10837>
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