Select all correct implementations of the or(x,y) function that for all argument expressions x and y , it evaluates same as x || y boolean expression.
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  • Function parameters can be passed by value or be passed by name.
    The same distinction applies to definitions.
    The def form is “by-name”, its right hand side is evaluated on each use.
    There is also a val form, which is “by-value”.
    val x = 2
    val y = square(x)
    The right-hand side of a val definition is evaluated at the point of the definition itself.
    Afterwards, the name refers to the value. For instance, y above refers to 4, not square(2).
  • The difference between val and def becomes apparent when the right hand side does not terminate.
    def loop: Boolean = loop
    A definition
    def x = loop
    is OK, but a definition
    val x = loop
    will lead to an infinite loop.
  • Boolean expressions b can be composed of
    true false // Constants
    !b // Negation
    b && b // Conjunction
    b || b // Disjunction
    and of the usual comparison operations:
    e <= e, e >= e, e < e, e > e, e == e, e != e
    Here are reduction rules for Boolean expressions (e is an arbitrary expression):
    !true --> false
    !false --> true
    true && e --> e
    false && e --> false
    true || e --> true
    false || e --> e
    Note that && and || do not always need their right operand to be evaluated.
    We say, these expressions use “short-circuit evaluation”.
  • To express choosing between two alternatives, Scala has a conditional expression if-else. It looks like a if-else in Java, but is used for expressions, not statements.
    def abs(x: Int) = if (x >= 0) x else -x
    x >= 0 is a predicate, of type Boolean.
    Here are reduction rules for conditional expressions (e1 and e2 are arbitrary expressions, b is boolean expression)
    if (b) e1 else e2:
    if (true) e1 else e2   --> e1
    if (false) e1 else e2  --> e2

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