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This library divides the task of parsing EDN into two stages. In the
first stage, implemented by `(EDN:READ-EDN string)` an EDN file is is
parsed into an AST where primitives are converted into lisp values and
compound forms are converted into lists of the form
`(type-specifier . data)`. This AST can be passed to
`(EDN:SYNTHESIZE implementation ast)` to produce datastructures of a
specific kind. The system `cl-edn/fset` provides a synthesizer that
produces appropriate fset datastructures. The system
`cl-edn/fset-lossy` produces fset datastructures, but forces symbols
and keywords to have uppercase names, for easier interoperation with
CL's default readtable. As a convenience, there is also
`(EDN:PARSE string &optional (implementation 'fset))` that combines
the two steps into a single call. These implementations can passed to
`PARSE` and `SYNTHESIZE` either as the symbols `EDN:FSET` and
`EDN:FSET-LOSSY` or by instantiating the classes named by those symbols.
EXTENSION
=========
`EDN:SYNTHESIZE` is a generic function that takes an implementation as
the first argument. The main system, `CL-EDN`, provides two
implementations of this function: one that specializes the first
argument on `SYMBOL`, that just makes an instance of the class named
by `IMPLEMENTATION` and calls `EDN:SYNTHESIZE` with the instance as
its first argument; the other implementation inspects the second
argument and, if it is a list, it delegates to
`(EDN:SYNTHESIZE-COMPOUND IMPLEMENTATION DISCRIMINATOR ARGS)`, the
head of the list as `DISCRIMINATOR` and the tail as `ARGS`. The
default implementation of this generic also provides methods for
strings, symbols and keywords that produce the relevant lisp types, as
well as an implementation for tagged literals that implements `#inst`
and `#uuid` and produces a form `(:TAGGED TAG-SYMBOL DATA)` where
`DATA` is synthesized according to the rules governing
`IMPLEMENTATION` (e.g. the `FSET` implementation makes `DATA` to be an
instance of the datastructures provided by the `FSET` library). Tags
can also be added to the default implementation by providing an
EQL-specialized method of `(EDN:SYNTHESIZE-TAG IMPLEMENTATION TAG ARG)`
for the symbol you want to define a behavior for. When this method is
called, the tag-symbol will be uppercased and converted into a common
lisp keyword and the `ARG` will have been synthesized according to the
rules provided by `IMPLEMENTATION`. To override this behavior, an
implementation can override `EDN:SYNTHESIZE-COMPOUND`, but such
implementations should either call `CALL-NEXT-METHOD` or implement
`#inst` and `#uuid` processing themselves.
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