#define XCONCAT(a, b) a##b
#define CONCAT(a, b) XCONCAT(a, b)
#define PS(val) () <- trace (val) (return ())
#define VIS(ioaction) let CONCAT(b, __LINE__) = unsafeVisualize(ioaction)
#define V(ioaction) CONCAT(b, __LINE__) = unsafeVisualize(ioaction)

V(foo)