I want to assert statically assert equality of polymorphic parameters using LiquidHaskell. See for instance the following simplified Array type containing an array shape and for demonstration purposes only a single element: data Array sh a = Array {shape :: sh, element :: a} {-@ lift2 :: (Eq sh) => (a -> b -> c) -> arrA : Array sh a -> {arrB : Array sh b | shape arrA == shape arrB} -> Array sh c @-} lift2 :: (Eq sh) => (a -> b -> c) -> Array sh a -> Array sh b -> Array sh c lift2 f (Array sha a) (Array shb b) = if sha == shb then Array sha $ f a b else error "shapes mismatch" Running liquidhaskell yields Illegal type specification ... Sort Error in Refinement: {arrB : (Example.Equality.Array sh##a31s b##a31u) | Example.Equality.shape arrA == Example.Equality.shape arrB} Unbound symbol Example.Equality.shape I think the Haskell function (==) is not automatically lifted to LiquidHaskell's logic language. Is there another way to assert a kind of equality in LiquidHaskell?