guard跟在函数名和参数之后，用管道符号表示：
myCompare :: (Ord a) => a -> a -> Ordering
myCompare a b
| a < b = LT
| a == b = EQ
| otherwise = GT
guard长得和pattern类似，pattern检查输入是否符合这个pattern，而guard则会计算布尔表达式，返回True或False。如果返回值是True，则执行等号后面的函数体部分。
看另外一个例子：
The first pattern specifies that if we try to take a 0 or negative number of elements, we get an empty list. Notice that we’re using _ to match the list because we don’t really care what it is in this case. Also notice that we use a guard, but without an otherwise part. That means that if n turns out to be more than 0, the matching will fall through to the next pattern.
当guard没有otherwise，如果都不匹配的话，就会执行下一个pattern。
参考资料：
Guards, guards!。