I'm in the process of learning Clojure and I can't understand some language design decisions: Why does a language with immutable Strings like Clojure also needs Keywords and Symbols data types? Couldn't strings just have optional namespaces and metadata and all this stuff? For immutable strings comparison could just as well be identity base, no?
Or, since interop with Java is a must have for Clojure, at least have the Java String type and a KeywordSymbol data type.
I find this String/Keyword/Symbol "trichotomy" especially weird since Clojure seems very focused on "purity" and keeping things simple in other aspects.
In Common Lisp, a "symbol" is a location in memory, a place where data can be stored. The "value" of a symbol is the data stored at that location in memory. In Clojure, a "symbol" is just a name. It has no value.
Keywords are symbolic identifiers that evaluate to themselves. They provide very fast equality tests... Symbols are identifiers that are normally used to refer to something else. They can be used in program forms to refer to function parameters, let bindings, class names and global vars...
They fill very different roles within the language:
runnable
and can be used directly to invoke functions. You cannot run a string.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
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