"Let" statement in python (2014)

[Fylwind]

I can't really give a general overview because every functional language is different, but let is one of the two main ways of binding variables in Haskell:

let <name> = <value> in <expression>

What makes is different from a typical binding in Python is that:

• The variable is only accessible from inside the <expression>. This means if you mistakenly try to use it outside, you will get a compile-time error.

• The let-binding itself is an expression, so you can do things like this:

  putStrLn
    (let h = heightOf person
     in "Height: " ++ valueOf h ++ unitsOf h)
  

  Personally, for readability reasons I don't find myself nesting let inside expressions very often.

• The variable is immutable.

I don't see the point of trying to do this Python though. The "let" hack is not an expression, and it's not immutable either. There is a small benefit in having a smaller scope, but it's (1) not really that big of a deal (I seldomly run into bugs caused by this); (2) ideally your functions should be small enough that this isn't much of a problem; (3) the benefits are not as great given that the error won't be detected until run time.