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Author: sharpchen

docs/document/Skill/PowerShell/docs/Language/Expression.md

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+# Expression
+
+## Expression Grouping
+
+`()` is grouping expression that allows **assignment**/add higher execution **precedence** within it and **pass through the value to outer context.**
+It's worth noting that `()` does not allow to eval a control flow statement which is very counter-intuitive.
+
+```ps1
+# enforce precedence
+(1 + 2) * 3 # 9
+# inline assignment
+($foo = 1) # 1
+# eval a statement
+($null = if ($true) { "foo" }) # foo
+
+(if ($true) { "foo" }) # parsing error! # [!code error]
+```
+
+> [!IMPORTANT]
+> `()` only allows one single expression within it.
+
+## Sub-Expression
+
+`$()` is yet another way to eval a expression to value but suppress the output of assignment statement.
+
+```ps1
+$(if ($true) { "foo" }) # foo
+
+# assignment does not pass through value to outer context
+$($foo = 1 -gt 3) # $null
+
+# self increment is another special kind of assignment
+$($i++) # $null
+```
+
+Since `$()` has a nature of suppressing value from assignment statement, we can use it as a single calculation block.
+
+```ps1
+$foo = 1
+# custom calculation block
+$(
+    $foo = 1 + 2
+    # self operation is also suppressed even though it should output
+    $foo++
+
+    $foo
+    $foo # yield more than once
+).Length # 2
+```
+
+### Array Sub-Expression
+
+`@()` is almost the same as `$()` but ensures the output as an array, no matter how few items were popped from the expression.
+
+```ps1
+@($pwd).GetType().Name # Object[]
+$($pwd).GetType().Name # PathInfo
+```