It's useful because the read-string in core is passing a string to the reader, essentially like an eval[1].
The edn namespace is for Extensible Data Notation[2], and exports functions for reading an object from a stream and from a string without passing it to the lisp reader.[3]
Performance differences are likely not that critical between reading small amounts of data using string I/O vs reading small amounts of data using a library routine that reads and evaluates the data. Runtime evaluation of a string as a language expression is often available in interpreted languages like Python, Ruby, or the Lisp family, and these languages are not usually used in the most performance sensitive programs.
Three factors may make a difference between using the two aforementioned functions. First, performance can matter if the program is doing enough I/O. Secondly, the machinery needed to evaluate language expressions will have to be available at runtime, meaning that the executable will have to be interpreted by an interpreter capable of doing the evaluation (which Lisp, Python, etc. are—they have REPLs after all) or the executable in languages without a runtime interpreter will have to be linked statically or dynamically with code that does the evaluation, making it resulting executable larger. Third, there is the really important reason that programs should avoid reading and evaluating their inputs as expressions: security.
A process runs code within a certain security context, generally with the same privileges as the user running the program. If the data input to the process can run expressions not found in the code, the program’s author can make few security guarantees about the results of running the program. Programs may receive input from the network and may even run at elevated security levels (e.g. setuid); these programs should not evaluate arbitrary input. For example, many security vulnerabilities come from database programs reading strings that are passed directly to the SQL interpreter. See the XKCD “Exploits of a Mom” [1].
Compilers for Lisp have been around in some form for many years—I can remember hearing Patrick Winston talking about them when I first learned Lisp in his class in 1973. There are both interpreters and compilers for many languages and translators that are in between that use JIT compilation at run time.
Lisp and it’s offshoots (Scheme, Dylan, Common Lisp, Clojure, etc.) have a wider range of compilation and interpretation strategies used in their implementations than other programming languages. Perhaps this is due to the fact that Lisp has been around longer than almost any other important language or perhaps its due to its homoiconicity.
However my point was really about the availability of eval at run time, however it is implemented, and particularly eval that runs in the program’s address space and security context.
It’s true that the SBCL implementation of Common Lisp, by default, compiles eval’s arguments at runtime, but it may also use interpretation at runtime depending on the value of the a global variable. Other implementations of Common Lisp always use interpretation, like CLISP. Also, Common Lisp has eval-when which affects the compile vs interpret decision [1].
Readers interested in interpretation vs compilation can find introductory coverage of the topic in Wikipedia, see [2]. Lisp has an interesting and important history to computer science as the first interpreted high level language, see it’s Wikipedia entry at [3].
Sure, just categorizing Lisp as an interpreted languages isn't correct.
> However my point was really about the availability of eval at run time, however it is implemented, and particularly eval that runs in the program’s address space and security context.
Right, but technically Common Lisp doesn't requite eval to use a compiler and it can also load compiled code from the file system, at runtime.
> Other implementations of Common Lisp always use interpretation, like CLISP.
CLISP also provides a virtual machine with a byte code compiler (also optionally a JIT native code compiler), though the compiler needs to be called explicitly.
> Also, Common Lisp has eval-when which affects the compile vs interpret decision [1].
Depends. In an implementation like SBCL or CCL, which use compilation for EVAL, it has no effect on such a decision.
