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Decision Making

Blade wouldn't be a complete usable language if all you can do is manipulating strings and numbers. Blade let's you do more. Blade has powerful features to help you make decisions, control the flow of executables and more like you'll expect in any programming language worth using at all.

For this chapter, we'll be focusing on the tools Blade has in it's arsenal for making decisions.

Reference

if Statements

From previous experience in other programming languages, the if statement is by far the most popular and used control flow in any programming language. For anyone coming from lanaguages like Java or C++, Blade's if statement will be directly intuitive to you and maybe even more familiar to a Go developer.

For example:

%> var a = 25
%> if a > 10 {
..   print('a is greater than 10')
.. }
a is greater than 10

Important: In a Blade block, the last statement can ommit the newline before closing the block with the right brace (})

Now, take a moment and consider the code above. For those familiar with Java, C++ or similar languages, you should note that unlike those languages, the if statement does not required parenthesis (() round it's condition, but it can be used if desired for grouping.

For example, we can rewrite the above if statement as follows:

%> if(a > 10) {
..   print('a is greater than 10')
.. }

@convention: Omit the parenthesis unless it make the expression clearer.

The if statement also support an else statement that must follow after it, which will execute when the condition in the if statement is not true.

For example:

%> var age = 93
%> if age < 65 {
..   echo 'User is not retired'
.. } else {
..   echo 'User is retired'
.. }
'User is retired'

The else statement can have another if statement as it's own block. For example:

%> if a < 18 {
..   echo 'User is a child'
.. } else if age > 65 {
..   echo 'User is retired'
.. } else {
..   echo 'User is working class'
.. }
'User is retired'

If you are only going to be having one statement in your if block, or any block in general, you can ommit the braces ({}) completely

For example:

%> if age == 93 echo 'User is 93'
'User is 93'

Another example that explores using a semi-colon (;) as a statement terminator:

%> if age == 5 echo 'User is 5'; else echo 'User is 93'
'User is 93'

Multiple conditions can be evaluated together using and and or as applicable to the statement. For example:

%> var a = 20
%> if a > 5 and a <= 20 {
..   echo 'Both conditions are true'
.. }
'Both conditions are true'
%> if a < 5 or a > 10 {
..   echo 'One of the condition is true'
.. }
'One of the condition is true'

Tenary Operators

The tenary (or conditional) operator like in most languages that supports it takes three operands.

  1. The condition followed by a question mark (?)
  2. An expression to evaluate if the condition is true, followed by a colon (:)
  3. An expression to evaluate when the condition is false.

For example:

%> 10 ? 'ok' : 'no'
'ok'
%> var a = 10 > 5 ? 21 : 19  # used to determine variable value
%> a
21

using Statements

The using statement in Blade is akin to the switch statement in C/C++ (or Java if you insist I mention it). When there are two or more predefined options to react to or decide on, the using statement offers superior speed when compared to multiple/nested if blocks. The emphasis here is on predefined.

using statements are simply lookup tables for a predefined set of options and code bound to each option. Each option must be defined using the when keyword followed by the option value, then a statement or block of code to execute. Blade will only execute a matching block or do nothing if no match is found.

For example:

%> var name = 'Larry'
%> using name {
..   when 'James' echo 'My name is James'
..   when 'Larry' echo 'My name is Larry'
.. }
'My name is Larry'

The values given to the when keyword must be constants, i.e. expressions such as 2 + 2 or using a variable will raise a syntax error. For example:

%> using true {
..   when(2 * 5) echo 'Bug'
.. }
SyntaxError at '(': only constants can be used in when expressions
  <repl>:2
SyntaxError at '}': expected expression
  <repl>:3

For those coming from the Java or C++, take notice that there is no use of the break keyword. In fact, it is a syntax error to use a break keyword here. For example,

%> using 1 {
..   when 1 break
.. }
SyntaxError at 'break': 'break' can only be used in a loop
  <repl>:2

The using statement also support a default case which will be executed if given whenever a match could not be found. For example:

%> var a = 25 
%> using a {
..   when true echo 'It\'s true'
..   when false echo 'I don\'t even understand'
..   default echo 'It\'s 25'
.. }
"It's 25"

To have more than one statement in the when option, simply use a block. For example:

%> var age = 30 
%> using age {
..   when 25 {
..     age++
..     echo 'Age increased'
..   }
..   default {
..     age--
..     echo 'Age reduced'
..   }
.. }
'Age reduced'



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