Bug #16092 ยป modifier-statements.patch
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expressions}[rdoc-ref:syntax/control_expressions.rdoc]
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if::
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Used for +if+ and modifier +if+ expressions. See {control
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Used for +if+ and modifier +if+ statements. See {control
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expressions}[rdoc-ref:syntax/control_expressions.rdoc]
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in::
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See {modules and classes}[rdoc-ref:syntax/modules_and_classes.rdoc]
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unless::
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Used for +unless+ and modifier +unless+ expressions. See {control
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Used for +unless+ and modifier +unless+ statements. See {control
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expressions}[rdoc-ref:syntax/control_expressions.rdoc]
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until::
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== Modifier +if+ and +unless+
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+if+ and +unless+ can also be used to modify an expression. When used as a
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modifier the left-hand side is the "then" expression and the right-hand side
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modifier the left-hand side is the "then" statement and the right-hand side
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is the "test" expression:
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a = 0
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This will print 0.
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While the modifier and standard versions have both a "test" expression and a
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"then" expression, they are not exact transformations of each other due to
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"then" statement, they are not exact transformations of each other due to
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parse order. Here is an example that shows the difference:
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p a if a = 0.zero?
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of a +rescue+ block. See {Exceptions}[rdoc-ref:syntax/exceptions.rdoc]
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for proper usage of +retry+.
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== Modifier Statements
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When used as a modifier, +if+, +else+, +while+, +until+ and +rescue+ become
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statements, not expressions. This means they cannot be used in contexts where
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an expression is expected, such as method arguments. Wrap with parentheses
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to create an expression.
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puts( 1 if true ) #=> SyntaxError
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puts((1 if true)) #=> 1
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puts (1 if true) #=> 1, because of optional parentheses for method
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puts( 1 rescue true ) #=> SyntaxError
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puts((1 rescue true)) #=> 1
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puts (1 rescue true) #=> 1, because of optional parentheses for method
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The right-hand side of a modifier statement is also one of those contexts
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where an expression is expected. So in <code>a if b rescue c</code>, because
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<code>b rescue c</code> is a statement and therefore not allowed as the
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condition of the +if+ statement, the code is necessarily parsed as <code>(a
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if b) rescue c</code>.
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This interacts with operator precedence in such a way that:
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stmt if v = expr rescue x
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stmt if v = expr unless x
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are parsed as:
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stmt if v = (expr rescue x)
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(stmt if v = expr) unless x
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== Flip-Flop
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The flip-flop is a rarely seen conditional expression. It's primary use is
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<code>-1</code> or <code>-(a + b)</code>.
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Modifier-if, modifier-unless, etc. are for the modifier versions of those
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keywords. For example, this is a modifier-unless expression:
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keywords. For example, this is a modifier-unless statement:
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a += 1 unless a.zero?
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Note that <code>(a if b rescue c)</code> is parsed as <code>((a if b) rescue
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c)</code> regardless of precedence. See {modifier
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statements}[control_expressions_rdoc.html#label-Modifier+Statements].
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<code>{ ... }</code> blocks have priority below all listed operations, but
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<code>do ... end</code> blocks have lower priority.
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