:4:17: error: • Couldn't match type ‘m’ with ‘(,) a0’ Expected: StateT Int ((,) a0) () Actual: StateT Int m () ‘m’ is an interactive-debugger skolem • In the first argument of ‘runStateT’, namely ‘_result’ In the second argument of ‘($)’, namely ‘runStateT _result 0’ In the expression: snd $ runStateT _result 0