It's a big patch, but the summary is that it's okay to use a pattern in
an erased position if either:
- the pattern can also be solved by unification (this is the same as
'dot patterns' for matching on non-constructor forms)
- the argument position is detaggable w.r.t. non-erased arguments, which
means we can tell which pattern it is without pattern matching
The second case, in particular, means we can still pattern match on
proof terms which turn out to be irrelevant, especially Refl.
Fixes#178
This required a small change to auto implicit search (and I'm still not
sure about this). Now search arguments right to left, because solving
later arguments may resolve earlier arguments by unification and this
can happen in particular when chasing parent interfaces (which may have
fewer parameters).
Elaborate via either === (homogeneous equality) or ~=~ (heterogeneous
equality) both of which are synonyms for Equal. This is to get the Idris
1 behaviour that equality is homogeneous by default to reduce the need
for type annotations, but heterogeneous if that doesn't work.
There's a bit of a trade off here. It would be better to report the
ambiguity but this would lead to a need for (I think) excessive
precision in types which would impact usability. It will always take the
leftmost interface.
Chapter 7 tests added.
Idris 1 will fill in the last metavariables by matching rather than
unification, as a convenience. I still think this is okay, even if it's
a bit hacky, because it's a huge convenience and doesn't affect other
unification problems.
Also abstract over lets in guesses, like in delayed elaborators, to
avoid any difficulties when linearity checking and to make sure that let
bound things don't get reevaluated.
This is enough to get the Chapter 6 TypeDD tests working
Surprisingly == on Nat in the Idris prelude is linear! So shortcut that
by converting to an Integer first. Also a couple of small things in the
evaluator that have a small but noticeable effect when environments are
big.
