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Template Expressions

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• expressions syntax

expressions syntax

Template expressions respect a defined syntax.

Syntax

expression is:

{ conditional_expression
  | unary_op expression
  | expression op expression
  | expression_as_string in expression_as_string
  | [ argument_list ]
  | ( expression )
  | fct ( argument_list )
  | template_path
  | numeric
  | boolean
  | string
  | null
  | undefined
}

where conditional_expression is:

boolean_expression ? if_true : if_false

where unary_op is:

{ ! | + | - }

where op is:

{ arithmetic_op | boolean_op | comparison_op }

where arithmetic_op is:

{ * | / | % | + | - }

where comparison_op is:

{ < | <= | > | >= | == | != }

where boolean_op is:

{ && | || }

where boolean is:

{ TRUE | FALSE }

where string is:

' string '

where argument_list  is a list of expressions separated by comas:

{ [ expression [, ...] ] }
 

argument_list can be empty.

Notes

1. conditional_expression returns a value according to the result of a condition expression

   boolean_expression ? if_true : if_false

   If the boolean_expression results in true, the conditional_expression returns the if_true expression, otherwise if_false is returned.

   orientation=='horizontal'?'':' gRadioGroupVertical'  # produces an empty string if orientation equals 
                                                        # horizontal, otherwise returns gRadioGroupVertical

2. expression_as_string  is a string
3. [ argument_list ] defines an array

   [ 1, 2, 'a', 'b' ]      # defines an array with 4 elements

4. ( expression ) defines a priority in the processing of the expressions.

   (a + b) * c             # means expression 'a + b' will be processed before the result is multiplied by c

5. fct is any valid function name
6. string is a string value
7. numeric is a numeric value
8. expression_as_string is any template expression resulting in a string value
9. template_path is any valid template path
10. null expression removes a corresponding attribute

    title comment || null;  # if comment attribute is not an empty string 
                            # then set comment value to title otherwise 
                            # remove the html attribute title 

11. undefined is the value returned by a template path when the path is not available. It is used to make the difference between a template path that would return an empty string and a path that does not exist in the current context.
    
12. The + operator can be used with string values to concatenate two strings. Other operators will not work unless string values evaluate to numeric expressions:

    'This expression ' + 'works'
    '12' - '34'                       # works
    '12' - 'ab'                       # doesn't work

13. The boolean operators && and || behave like their JavaScript equivalent. The following table gives the result of the expression:

    expr_A op expr_B

    Operator	expr_A is TRUE*	expr_B is FALSE*
    &&	expr_B	false
    ||	expr_A	expr_B

    *Evaluation is done like the test made by the gwc:condition instruction

    1 > 0 && 'expr_A is true'         # produces expr_A is true
    1 < 0 && 'expr_A is false'        # produces 0
    1 < 0 || 'expr_A is false'        # produces expr_A is false

    As the && operator's priority is greater than the || operator's priority, you can combine these operators to have an if … then … else … statement :

    condition_expr && expr_if_true || expr_if_false

    true  && 'bill' || 'bob'          # produces bill
    false && 'bill' || 'bob'          # produces bob

14. The in operator is used to look for a string value in a string value list. Elements of the list are separated by a space character

'bill' in 'bob bill john'         # produces true

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