Haskell/Solutions/Laziness

Why must we fully evaluate x to normal form in f x y = show x? Which is the stricter function? f x = length [head x] g x = length (tail x) Because show x needs to evaluate x to convert it to a String. g is stricter: it must walk through the whole list to discover its length, even if it does not need to examine the single values. Instead, to evaluate length [head x], when length evaluates the passed list it obtains thunk_0:[], where thunk_0 represents the result of head x. Thus, you can verify that f undefined => 1 (actually, f is equivalent to const 1) while g undefined fails. Also note that, if ls is a valid Int list (which does not contain undefined among its elements), all following expressions fail to evaluate: g (1:undefined) => length undefined => undefined whereas the following evaluate successfully: g (1:undefined:ls) => length (undefined:ls) => 1 + length ls g (undefined:ls) => length ls The difference between g (1:undefined) and g (1:undefined:ls)may appear confusing, but is correct, because in the first case undefined replaces a list (in particular, the tail of the cons cell passed to g), while in the second case undefined replaces a list value (which is not needed).