Meaning of "let" and "in"

Learning
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This is actually even more important: For a clear semantic we need some way to distinguish declaration and assignment: What does a = 42 mean? Is it:

  • Creating a new binding a with the value 42?
  • Assigning the existing a with the value 42?

Python got this wrong, where the compiler doesn’t know and where it is ambiguous would exit with an UndefinedLocalError. So they had to add global and nonlocal as declaration to tell the compiler what the meaning of the subsequent a = 42 is.

So yes, it is less writing, but the let is imporant. One could attempt to avoid the in and make it implied (something like let a = b and b = 42 a) but it can be somewhat confusing.

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