For example, the following Python code has the same evaluation semantics.
def any_predicate(predicate, iterable):
return any(map(predicate, iterable))
However, It can lead to hard to debug errors (as the error will not necessarily surface at the call site), and make it hard to reason about the performance and memory usage of code.
One of the positive side effects of this fact is that Haskellers have been forced to figure out proper error handling strategies, for the exact same reason Haskellers were forced to figure out proper effect management strategies (e.g. monadic IO). In fact, errors and effects are part of the same problem.
In imperative languages, the temptation to use evaluation sequencing as a hack for handling errors and effects has proven too tempting for every pre-Haskell language I'm aware of.
Agreed. Clojure uses lazy sequence by default. After ClojureScript is compiled to js, sometimes it's crazily hard to figure out why the error happened due to the obscure call site in the code.
Good point. Interestingly, there doesn’t seem to be a commonly known notion of evaluation that would blur the distinction between FEXPRs and functions — that is, one that would allow “functions” access to the unevaluated syntactic form of the arguments.
I found this article really helpful for explaining why laziness is important: http://augustss.blogspot.com/2011/05/more-points-for-lazy-ev...
In particular, scroll down to the "Reuse" section. Lazy evaluation enables you to compose functions to build new functions with the right performance characteristics (constant factors aside), as in:
In an eager language, predicate `p` would have to be evaluated on every element in the list, even if the predicate is already satisfied after the first element. So instead you'd have to write this function in an ad hoc way with explicit recursion/looping, without being able to reuse the more primitive functions.Of course, in an impure language, laziness can't be the default because you wouldn't be able to reason about when your side effects occur. But in my mind that's an argument for purity rather than an argument for eagerness.
BTW, comparing "strict" vs. "lazy" is a category error. The proper comparison would be either "strict" vs. "non-strict" or "eager" vs. "lazy". The former is in reference to the denotational semantics, whereas the latter characterizes the operational semantics.