[GHC] #8113: Cannot override ghci builtin commands with :def[!]

[GHC]

#8113: Cannot override ghci builtin commands with :def[!]
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        Reporter:  duncan            |            Owner:
            Type:  bug               |           Status:  closed
        Priority:  normal            |        Milestone:  7.8.1
       Component:  GHCi              |          Version:  7.6.3
      Resolution:  fixed             |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  Other             |       Difficulty:  Easy (less than 1
       Test Case:                    |  hour)
  ghci/scripts/T8113                 |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by rwbarton):

 I suppose what I specifically want to happen when I enter a `:command` is
 an algorithm like this.

 If the name I entered is an exact match for a macro or built-in, use that
 name.

 Otherwise, try to complete the name to the name of a ''built-in'' in the
 traditional way. If this succeeds, use the resulting name.

 Otherwise, try to complete the name to the name of a macro, and use the
 resulting name if that succeeds, otherwise give up.

 In all cases where we got a name, use the ''macro'' of that name if there
 is one, and otherwise use the built-in.  (Obviously, for `::command`,
 ignore macros entirely.)

 In other words, built-ins should take precedence over macros for the
 purpose of name ''completion'', but macros should take precedence over
 built-ins for the purpose of name ''lookup''.  This is backwards-
 compatible from the perspective of the user who is not aware of the
 change—`:t` will always mean `:type`, as long as the user has no macro
 named `:t`, just like in previous versions of ghci—while still allowing
 the aware user to redefine exactly what `:type` means.  And it's flexible
 enough in that if the user really wants `:t` to complete to some other
 macro `:test` that they've written, they can always define another macro
 `:t` to expand to `:test`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8113#comment:14>
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