prependToSubst: Eric broke unification. Prepending K-t-73 twice about geni

[kowey]

Imported from trac issue 31. Created by kowey on 1970-01-01T01:20:53, last modified: 1970-01-01T01:20:53

[kowey]

Trac comment by kowey on 1970-01-01 01:20:53:

It seems to be t150 in the XMG-GenI integration demo

[kowey]

Trac comment by kowey on 1970-01-01 01:20:53:

In the debugger, it happens at iteration 142 (or 141?)

[kowey]

Trac comment by kowey on 1970-01-01 01:20:53:

I can confirm that GenI 0.17.4 makes it through this.
So either this is a regression or something else has changed that reveals the error.

If I understand correctly, this bug means we're trying to register more than one substitution for a variable, which doesn't make sense because normally once you've registered a substitution that variable is gone. I think this means that we're basically just failing to propagate substitutions somewhere...

This makes me want a better abstraction over unification.

[kowey]

Trac comment by kowey on 1970-01-01 01:20:53:

Cool! This may not be a unification bug after all, but a repeated adjunction.
Why are we getting this?

[kowey]

Trac comment by kowey on 1970-01-01 01:20:53:

Resolved by:
Fri Sep 25 17:16:00 CEST 2009 Eric Kow [email protected]

• Fix #31 - unification bug cause by failure to propagate.

  We do top and bottom unification after adjunction; however, the
  nodes that we were doing unification on did not have the latest
  substitutions from the adjunction test applied to them.

  I'm still not sure why this is a regression from GenI 0.17, but
  that's a future question to research.

  Note also that this bug would not have been caught without the
  pattern-matching style allowed by languages such as Haskell or one of
  the MLs. We simply would have silently accepted the anomalous situation
  and the user would have been none the wiser. I consider this a
  succesful application of the "Dead Programs Don't Lie" principle.
  So yay Haskell!