As Kent Beck notes when discussing guard clauses in Implementation Patterns making a routine have a single entry and exit point ...
"was to prevent the confusion possible
when jumping into and out of many
locations in the same routine. It made
good sense when applied to FORTRAN or
assembly language programs written
with lots of global data where even
understanding which statements were
executed was hard work ... with small methods and mostly local data, it is needlessly conservative."
I find a function written with guard clauses much easier to follow than one long nested bunch of if then else statements.
