[GHC] #1896: Keep old bindings until :load succeeds

[GHC]

#1896: Keep old bindings until :load succeeds
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 Reporter:  tibbe            |          Owner:          
     Type:  feature request  |         Status:  new     
 Priority:  normal           |      Milestone:          
Component:  GHCi             |        Version:          
 Severity:  normal           |       Keywords:          
 Testcase:                   |   Architecture:  Multiple
       Os:  Multiple         |  
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 Imagine you make a change to a module which is already loaded into GHCi
 and then try to {{{:load}}} it again. If the load fails not only will you
 not have the new bindings but the old ones are lost as well. This prevents
 you from e.g. using {{{:type}}} to figure out why your new change doesn't
 type check.

 A more ambitious solution would be to try to incrementally load the module
 and keep all bindings up to the one that fails (and its dependents.) This
 would allow you to debug the failure more interactively (e.g. compose
 functions and look at their combined types.) This is probably trickier as
 previous bindings might be invalidated.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1896>
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