Main.hs

newtype Box phantom = Box Bool

unbox :: Box phantom -> Bool
unbox (Box content) = content

myBox :: Show phantom => Box phantom
myBox = Box False

result :: Bool
result = unbox myBox

--     • Ambiguous type variable ‘phantom0’ arising from a use of ‘myBox’
--       prevents the constraint ‘(Show phantom0)’ from being solved.
--       Probable fix: use a type annotation to specify what ‘phantom0’ should be.
--     • In the first argument of ‘unbox’, namely ‘myBox’
--       In the expression: unbox myBox
--       In an equation for ‘result’: result = unbox myBox
--    |
-- 10 | result = unbox myBox
--    |                ^^^^^