[ME-0002] logical not operator

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proposals/0002-logical-not-operator.mbt.md

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+# Logical not operator
+
+* Proposal: [ME-0002](https://github.com/moonbitlang/moonbit-evolution/blob/0002-orsuccess/proposals/0002-logical-not-operator.mbt.md)
+* Author: [Yorkin](https://github.com/Yoorkin)
+* Review and discussion: [Github issue](https://github.com/moonbitlang/moonbit-evolution/pull/3)
+
+## Introduction
+
+In MoonBit, the `not` function takes a boolean value and returns its negation, 
+as implemented in `moonbitlang/core` . We propose the `!expr` syntax to negate a 
+boolean expression, which aligns with the conventions of other programming languages.
+
+## Motivation
+
+Since `not` is a function, boolean expressions must be wrapped in parentheses 
+when used. This can be cumbersome and deviates from the conventions of other 
+programming languages that use a logical operator for negation. Furthermore, 
+we already have `&&` and `||` for logical operations. This inconsistency may 
+confuse users from diverse programming backgrounds and lead to misunderstandings 
+by LLMs during code generation.
+
+## Solution
+
+We propose introducing a logical operator `!` that can be used in expressions 
+to negate a boolean value:
+
+```moonbit
+fn f(x : Bool) -> String {
+  if !x {
+    "false"
+  } else {
+    "true"
+  }
+}
+
+let false_value = !true
+let true_value = !!true // !(!true)
+```
+
+The `not` function will still be available for pipeline syntax and use as a 
+higher-order function.