[GHC] #9334: Implement "instance chains"

[GHC]

#9334: Implement "instance chains"
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              Reporter:  diatchki    |            Owner:  diatchki
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by aavogt):

 Replying to [comment:6 diatchki]:
 >   2. "Fails instances", which are of the form `instance Num Char fails`;
 they enable programmers to state explicitly that an instance should never
 exits.  Interestingly, I just found a very related ticket asking for the
 same sort of thing (#7775).

 In some sense this can already be done:
 {{{
 class FailHasNoInstances a => Fail a
 class FailHasNoInstances a -- not exported to ban Fail instances

 instance Fail "Char may not have a Num instance" => Num Char
 main = print $ '1' + '1'
 {- has a compile failure:
     No instance for (Fail "Char may not have a Num instance")
       arising from a use of ‘+’
     In the second argument of ‘($)’, namely ‘'1' + '1'’
     In the expression: print $ '1' + '1'
     In an equation for ‘main’: main = print $ '1' + '1'
 -}
 }}}

 >   3. "Using instance contexts when selecting instances (aka
 backtracking)": currently, if the head of an instance matches a goal, GHC
 commits to it and then fails if it encounters an error;  an alternative
 design would be to back-track and try a different option (e.g., next
 member of an instance group, or a more general matching instance).

 Perhaps with a class like
 {{{
 class HasInstance (cxt :: Constraint) (b :: Bool) | cxt -> b
 }}}
 you can encode backtracking without too much pain, and the meaning of
 existing programs does not change.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9334#comment:9>
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