I don't think this is very functional. First of all, the let binding inside the condition has type bool by itself (instead of the normal let-in construction). A

I don't think this is very functional. First of all, the let binding inside the condition has type bool by itself (instead of the normal let-in construction). Also, it's scope permeates into the body of the if condition. The whole thing is very much like C-style NULL/Not-NULL check: if (x = proc()) {printf("%i", x);}