With-clause doesn't take advantage of ruled out patterns to normalise about idris2

[edwinb]

Comment by edwinb
Saturday Dec 07, 2019 at 15:11 GMT

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This'd be nice, but would require the normaliser to keep track of a lot of information. I think this is a feature request that's very unlikely to get done, unfortunately. I've added a label, just in case anyone is enthusiastic enough to do it, but my feeling is that it'd be very complicated to achieve.

(My feeling on this sort of thing is often wrong though, so who knows...)

[gallais]

For the record, the typical solution here is to define a view
that has one case for each match of interest & a catchall
constructor for the rest.

Instead of:

foo : s -> ...
foo x = case x of 
  C => ...
  _ => ...

write:

data View s = C | Default

view : (x : s) -> View x
view x = case x of
  C => C
  _ => Default

foo : s -> ...
foo x with (view x)
  foo x | C = ...
  foo x | Default = ...

and you can then prove foo's properties by using the same with (view x).