1/1: Building param (param.idr)
1/1: Building parambad (parambad.idr)
Error: While processing right hand side of U. Name Main.X.foo is private.

parambad:7:7--7:10
 3 |     foo : Bool
 4 |     foo = True
 5 | 
 6 |   U : Bool
 7 |   U = foo
           ^^^

Suggestion: add an explicit export or public export modifier. By default, all names are private in namespace blocks.