A lambda is just an anonymous function - a function defined with no name. In some languages, such as Scheme, they are equivalent to named functions. In fact, the function definition is re-written as binding a lambda to a variable internally. In other languages, like Python, there are some (rather needless) distinctions between them, but they behave the same way otherwise.
A closure is any function which closes over the environment in which it was defined. This means that it can access variables not in its parameter list. Examples:
def func(): return h
def anotherfunc(h):
return func()
This will cause an error, because func
does not close over the environment in anotherfunc
- h
is undefined. func
only closes over the global environment. This will work:
def anotherfunc(h):
def func(): return h
return func()
Because here, func
is defined in anotherfunc
, and in python 2.3 and greater (or some number like this) when they almost got closures correct (mutation still doesn't work), this means that it closes over anotherfunc
's environment and can access variables inside of it. In Python 3.1+, mutation works too when using the nonlocal
keyword.
Another important point - func
will continue to close over anotherfunc
's environment even when it's no longer being evaluated in anotherfunc
. This code will also work:
def anotherfunc(h):
def func(): return h
return func
print anotherfunc(10)()
This will print 10.
This, as you notice, has nothing to do with lambdas - they are two different (although related) concepts.
Case classes can be seen as plain and immutable data-holding objects that should exclusively depend on their constructor arguments.
This functional concept allows us to
- use a compact initialization syntax (
Node(1, Leaf(2), None))
)
- decompose them using pattern matching
- have equality comparisons implicitly defined
In combination with inheritance, case classes are used to mimic algebraic datatypes.
If an object performs stateful computations on the inside or exhibits other kinds of complex behaviour, it should be an ordinary class.
Best Answer
In addition to what Lee said, you can define
reduce
in terms offold
, but not (easily) the other way round:The fact that
fold
takes an explicit initial value for the accumulator also means that the result of thefold
function can have a different type than the type of values in the list. For example, you can use accumulator of typestring
to concatenate all numbers in a list into a textual representation:When using
reduce
, the type of accumulator is the same as the type of values in the list - this means that if you have a list of numbers, the result will have to be a number. To implement the previous sample, you'd have to convert the numbers tostring
first and then accumulate: