Help on first ticket

Simon Peyton Jones simonpj at
Fri Jun 24 11:20:15 UTC 2016

Richard, did you get on ok?


|  -----Original Message-----
|  From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Ben
|  Gamari
|  Sent: 04 June 2016 23:20
|  To: Richard Fung <minesasecret at>; ghc-devs at
|  Subject: Re: Help on first ticket
|  Richard Fung <minesasecret at> writes:
|  > Hello! I apologize if this isn't the right place to ask; if it isn't
|  > please steer me in the right direction.
|  >
|  Hi Richard!
|  > Would anyone be willing to advise me on my first ticket? I've been
|  > trying to work on it on and off but haven't made much progress on my
|  own.
|  >
|  > It's ticket #9370:
|  >
|  Great, I'm happy to hear that someone has picked this one up. I think
|  it is a nice choice for a self-contained newcomers project.
|  > I think I understand the issue conceptually but I don't know where
|  to
|  > look for the code that needs to be changed..
|  >
|  I don't know where the code responsible for this is off the top of my
|  head, however I can provide some pointers.
|  So the unfoldings you are looking to preserve come from interface
|  files.
|  The machinery for all of this is in compiler/iface. IfaceSyn.hs is of
|  particular interest and there you will find the definition of
|  IfaceUnfolding, which is the unfolding representation which is stored
|  in the interface file. Unfoldings live inside of IdInfo values, which
|  hold various miscellaneous information which we need to preserve about
|  a particular Id (identifier).
|  There is a somewhat useful comment regarding how IdInfo is treated
|  above the definition of IfaceIdInfo in IfaceSyn. In particular it
|  seems that interface files for modules compiled with -O0 will have
|  their IdInfo fields set to NoInfo. It's not clear what happens when an
|  interface file is read. However, grepping for NoInfo reveals a use-
|  site in TcIface.tcIdInfo which looks interesting (in particular the
|  ignore_prags guard). I think this should be enough to get you going on
|  the interface file part of this.
|  The other part of this ticket is deciding whether to use an unfolding
|  when considering whether to inline. This will be done in the
|  simplifier (compiler/simplCore). Grepping for "inline" and "unfold" in
|  simplCore/Simplify.hs (as well as reading the notes in that file) will
|  likely be enough to get you started.
|  Do let me know if you still feel lost or want to discuss this further.
|  I look forward to hearing how it goes.
|  Cheers,
|  - Ben
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