Szathmáry László honlapja @ DEIK | Nim2 / How an immutable string differs from Python

In Python, a string is immutable. In Nim, a let string is also immutable. But there is an important difference:

The diff
s = "aaaa" # s[0] = "B" # ERROR: immutable # The string is immutable => the string cannot be changed. # However, `s` is a reference and it CAN point # to another string object: s = "bbbb" print(s) # bbbb	let s = "aaaa" #s[0] = 'B' # ERROR: immutable #s = "bbbb" # ERROR # The string itself is protected, cannot be changed, # and `s` is also protected: it cannot point # to another string. #let s = "bbbb" # ERROR: cannot be redefined

In Python, s is a reference (a pointer) that can point anywhere. However, the pointed string is protected; the pointed string cannot be modified.

In Nim, when you make a let variable, and the type of the variable is value type, then everything is protected. The pointed value cannot be modified, and the variable cannot "point" anywhere else.

More accurately: in Nim, you bind a variable name to a value. The value is protected and cannot be modified, and the name cannot be rebound to another value. (You get this double protection for value types).