[Haskell-cafe] Terminology: different levels of strictness

[Yves Parès]

If I consider the functions head, length, elem & sum, each is of them is
strict, as:
head/length/elem x/sum _|_ are always _|_.

However:
head (x:_|_) is never _|_.
length [_|_, _|_, _|_ ...] is also never _|_.
elem x [4,5,6,8,2,90,_|_,_|_ ...] is *only sometimes *_|_ (depending on x
value).
In fact, only
sum [4,5,6,8,2,90,_|_,_|_ ...] is always _|_.

Which shows they don't have the same level of strictness.

So can you say things like "all these functions are strict, but some are *more
*than other", or "sum is *deeply strict*" ...?
What terms can you use to compare those functions?
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