RE: strange behaviour

It's correct behaviour to print "test" any number of times. Haskell is non-strict, which only means that things aren't evaluated unless needed. It's not (defined to be) lazy, which would mean that named expressions would be evaluated at most once (though ghc meets this). It's also not defined to be "fully lazy" meaning that unnamed expressions would be evaluated at most once in any given closure. So GHC is entirely within its rights to evaluate <> any number of times, or possible even none, since it knows that that expression always returns the same value ().

It wouldn't be correct to not evaluate it at all: it might be _|_ or raise an exception. Cheers, Simon