Compiler introducing extra interface requirements when using functions returning dependent pairs - idris

Question

This is rather complicated. Sorry I could'n make the example simpler. I'm trying to formalize a theory and interfaces A and B represent my axioms. X and Y are some objects in the theory, mkY creates a Y from two Xs and PropA, PropY and PropYY are some statements about these objects:
interface A a where
PropA : a -> Type
interface B where
X : Type
Y : Type
PropY : Y -> Type
mkY : A X => (x, y : X) -> (z : Y ** PropY z)
PropYY : Y -> Type
mkPropY : A X => {x : X} -> {y : Y} -> PropA x -> PropY y -> PropYY y
lemma1 : (B, A X) => (x, y : X) -> PropA x -> (z : Y ** PropYY z)
lemma1 x y prop_a =
let (z ** propZ) = mkY x y in
(z ** mkPropY prop_a propZ)
Unfortunately rather obvious lemma1 does not compile:
When checking right hand side of Example.case block in lemma1 at /home/sven/code/idris/geometry/src/Euclid/Example.idr:17:9-20 with expected type
(z1 : Y ** PropYY z1)
When checking an application of function Example.mkPropY:
Type mismatch between
PropA x1 (Type of prop_a)
and
PropA x (Expected type)
It seems to me Idris refuses to unify requirement A X from the function header with that introduced by mkY function. When I replace mkPropY prop_a propZ with a hole and ask for its type, I get this:
constraint : B
z : Y
propZ : PropY z
x : X
y : X
constraint1 : A X
prop_a : PropA x
constraint2 : A X
--------------------------------------
whatIsIt : PropYY z
Here constraint1 and constraint2 are the same, and yet there's two of them, which seems to be the root cause of the problem. So why does Idris introduce this additional constraint and how do I make it work?

Answer 1

I'm not sure why Idris thinks with mkY there could be another constraint in play (as by the looks of it nothing in the hole is constrained by it, even with :set showimplicits). Maybe someone else can explain why, but for now it usually helps to make constraints explicit:
lemma1 : (B, a_const : A X) => (x, y : X) -> PropA x -> (z : Y ** PropYY z)
lemma1 #{a_const} x y prop_a =
let (z ** propZ) = mkY #{a_const} x y in
(z ** mkPropY prop_a propZ)
(Maybe rewriting B as interface B x y where; … would help, so the scope over X and Y is clearer, but I didn't try this.)