Local Bindings¶

A much more interesting - but also complicated - topic is the local binding, which is done with the let expression and its relatives. The primary use case for local binding is the reuse of evaluations. When the result of a complex expression is used more than once it is more efficient and gives a cleaner input structure to create a local binding for the result and (re)use that.

Colloquially spoken you can say that let allows you to create one or more local variables and then evaluate one or more expressions. But in fact let represents exactly one expression with some local bindings and an expression body. The let expression itself evaluates to a value that can then be used externally.

With let we finally reach the point where the nested parentheses in Scheme can become pretty daunting, with lots of frightening and confusing error messages in the console. I remember very well how I desparately moved, removed and added random parens to make let expressions work, and once I managed to get them compile without errors I had no idea why and couldn't make use of the experience for future challenges. This was until I started understanding how consequently the expressions are structured and what each part is actually necessary for, and if you follow me through the next few chapters you will hopefully reach a comfortable level of familiarity pretty soon.