They fill very different roles within the language:
- Vars are used to give names to things. They implement
runnable
and can be used directly to invoke functions. You cannot run a string.
- Keywords are names by themselves, and look themselves up in maps. They really help Clojure keep its "data driven" flavor. Strings do not implement the required interfaces to look themselves up in maps.
- Strings are just strings. They do what they need to do and not much more.
One of the core principles in the design of Clojure was to embrace your host platform, thus in Clojure strings are Java strings and you never need to wrap a Java string in some convert-to-clojure-string
function in order to get it into the Clojure ecosystem. This necessitated using unmodified Java strings, as well as the numeric types. Keywords and symbols are new constructs that are being added by Clojure, so it is only necessary to make them accessible in a useful way from the rest of the Java ecosystem. Symbols and Keywords make themselves accessible by simply being classes that implement an interface. It was believed in the beginning that in order for a new language to succeed in the JVM ecosystem, it needed to fully embrace Java and minimise the "impedance mismatch" (sorry for the buzzwordism) even if that required adding more to the language than would have been required without this goal.
edit:
You can sort of turn a symbol into a keyword by def
ing it to it's self
user> a
; Evaluation aborted.
user> :a
:a
user> (def a 'a)
#'user/a
user> a
a
user>
keywords evaluate to themselves
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…