For prototyping convenience I've relied on a number of global variables which are heavily used throughout the code. But now I'd like to make some of them local (but dynamic). Is there any significant downside (eg, efficiency, something else) to declaring them locally special instead of global?
1 Answer
Unpopular features of special variables include:
- Lack of referential transparency
This makes it harder to reason functionally about your code. It means that your function produces different results with syntactically equivalent calls.
- Introduce bugs
If a lexical variable is defined somewhere in your code (eg. in a system function), you will overwrite it and cause bugs.
- Confusing
Special (dynamic) binding is unpopular, and will confuse your readers who are not familiar with it.
- Unnecessary
Just use lexical binding, or even anaphoric macros instead.
More information:
Anaphoric macros See Let Over Lambda by Doug Hoyte, or Paul Graham's anaphoric macros.
LiSP (Lisp in Small Pieces) has a section on binding and dynamic binding
Elisp has dynamic binding by default, and enforced dynamic binding for a long time
Many early lisps had dynamic binding, but dropped it.
1 Comment
davypough
Thank you for a comprehensive summary--all good reasons for not translating my global specials to local specials. Will investigate other ways to reduce dependence on globals (in addition to passing local versions as arguments).
specialvariables apply, obviously.