The body of a class or an object is the primary constructor. A constructor, like a method, is a sequence of statements that are executed in order — to do anything else, it would have to be a very different language. I’m pretty sure you wouldn’t like for Scala to execute the statements of your … Read more
We say Idris has strict evaluation, but this is for its run-time semantics. Being a fully dependently typed language, Idris has two phases where it evaluates things, compile-time and run-time. At compile-time it will only evaluate things which it knows to be total (i.e. terminating and covering all possible inputs) in order to keep type … Read more
String s = Stream.<Supplier<String>>of(this::first, this::second /*, … */) .map(Supplier::get) .filter(Objects::nonNull) .findFirst() .orElseGet(this::defaultOne); It stops on the first non-null value or else sets the value which is returned from defaultOne. As long as you stay sequential, you are safe. Of course this requires Java 8 or later. The reason why it stops on the first occurrence … Read more
With Ruby 1.9 you can use the Enumerator class. This is an example from the docs: fib = Enumerator.new { |y| a = b = 1 loop { y << a a, b = b, a + b } } p fib.take(10) #=> [1, 1, 2, 3, 5, 8, 13, 21, 34, 55] Also, this … Read more
You could use a @property on the metaclass instead: class MyMetaClass(type): @property def my_data(cls): if getattr(cls, ‘_MY_DATA’, None) is None: my_data = … # costly database call cls._MY_DATA = my_data return cls._MY_DATA class MyClass(metaclass=MyMetaClass): # … This makes my_data an attribute on the class, so the expensive database call is postponed until you try to … Read more
Yes, GHC’s RTS uses thunks to implement non-strict evaluation, and they use mutation under the hood, so they require some synchronisation. However, this is simplified due to the fact that most heap objects are immutable and functions are referentially transparent. In a multithreaded program, evaluation of a thunk proceeds as follows: The thunk is atomically† … Read more
Here’s an example > length (undefined : 3 : 4 : undefined : []) 4 > length (2 : 3 : 4 : 5 : undefined) <<loop>> The first list contains bottoms as elements, but the “shape” of the list is fully defined. Roughly speaking, every list cell has a clearly defined “pointer” to its … Read more
In addition to user239558’s answer, and in response to your comment there, I’d like to point out some tools that allow you to inspect the heap representation of your value, find answers to questions like this yourself and to see the effect of optimizations and different ways of compiling. ghc-datasize tells you the size of … Read more
Unboxed vs Boxed Data To support parametric polymorphism and laziness, by default Haskell data types are represented uniformly as a pointer to a closure on the heap, with a structure like this: (source: haskell.org) These are “boxed” values. An unboxed object is represented by the value itself directly, without any indirection or closure. Int is … Read more