README: library "Sexplib"
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Copyright (C) 2008 Jane Street Holding, LLC (1)
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Author: Markus Mottl
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New York, 2008-09-30
====================
1 Directory contents
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| CHANGES | History of code changes |
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| COPYRIGHT | Notes on copyright |
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| INSTALL | Short notes on compiling and |
| | installing the library |
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| LICENSE | "GNU LESSER GENERAL PUBLIC LICENSE" |
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| LICENSE.Tywith | License of Tywith, from which Sexplib is derived|
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| Makefile | Top Makefile |
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| OCamlMakefile | Generic Makefile for OCaml-projects |
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| OMakefile | Ignore this file |
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| README.txt | This file |
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| lib/ | OCaml-library for S-expression conversions |
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| lib_test/ | Test applications for the Sexplib-library |
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2 What is "Sexplib"?
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This library contains functionality for parsing and pretty-printing
S-expressions. In addition to that it contains an extremely useful
preprocessing module for Camlp4, which can be used to automatically generate
code from type definitions for efficiently converting OCaml-values to
S-expressions and vice versa. In combination with the parsing and
pretty-printing functionality this frees users from having to write their own
I/O-routines for datastructures they define. Possible errors during automatic
conversions from S-expressions to OCaml-values are reported in a very
human-readable way. Another module in the library allows you to extract and
replace sub-expressions in S-expressions.
3 How can you use it?
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Make sure you have installed the 'type-conv' package on your system, too. It
should be obtainable at the same site as 'sexplib'.
The API (.mli-files) in the 'sexplib' library directory is fully documented.
Module 'Sexp' contains all I/O-functions for S-expressions, module 'Conv'
helper functions for converting OCaml-values of standard types to
S-expressions. Module 'Path' supports sub-expression extraction and
substitution.
Module 'pa_sexp_conv.ml' contains the extensions for the
Camlp4-preprocessor. It adds the new construct 'with sexp' (and 'with sexp_of'
and 'with of_sexp', which are implied by the first). When using this construct
right after a type definition, function definitions will be generated
automatically, which perform S-expression conversions.
E.g. given the following type definition:
<< type t = A | B
with sexp
>>
The above will generate the functions 'sexp_of_t' and 't_of_sexp'. The
preprocessor also supports automatic addition of conversion functions to
signatures. Just add 'with sexp' to the type in a signature, and the
appropriate function signatures will be generated.
See the file 'lib\_test/conv\_test.ml' for example usage. It also
demonstrates how to extract and substitute sub-expressions.
3.1 Compiling and linking
==========================
To compile a file you will have to add a preprocessing flag to the compiler
invocation, e.g. for foo.ml:
<< ocamlc -pp "camlp4o -I {path to type-conv} \
-I {path to sexplib} pa_type_conv.cmo pa_sexp_conv.cmo" \
-I {path to sexplib} foo.ml
>>
If you are using OCamlMakefile, just put the following line at the top of
the file, assuming you have installed both 'type-conv' and 'sexplib' with
ocamlfind. The comment must start at the beginning of the line, and you must
not break lines (here broken for readability only):
<< (*pp camlp4o -I `ocamlfind query type-conv` \
-I `ocamlfind query sexplib` \
pa_type_conv.cmo pa_sexp_conv.cmo *)
>>
Don't forget to place the macro 'TYPE_CONV_PATH', which takes a string
argument, at the top of the file to be preprocessed (see example
'conv_test.ml'). It is supposed to set the module path to the given module
file. This is necessary, because modules may get packed at a later stage, and
error messages generated by Sexplib will refer to this location to easily find
the associated type.
In the linking stage you will only have to link with 'sexplib'. E.g. when
using OCamlMakefile just add it to the 'PACKS'-variable.
4 Syntax Specification of S-expressions
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4.1 Lexical conventions of S-expression
========================================
Whitespace, which consists of space, newline, carriage return, horizontal
tab and form feed, is ignored unless within an OCaml-string, where it is
treated according to OCaml-conventions. The semicolon introduces comments.
Comments are ignored, and range up to the next newline character. The left
parenthesis opens a new list, the right parenthesis closes it again. Lists can
be empty. The double quote denotes the beginning and end of a string following
the lexical conventions of OCaml (see OCaml-manual for details). All
characters other than double quotes, left- and right parentheses, and
whitespace are considered part of a contiguous string.
4.2 Grammar of S-expressions
=============================
S-expressions are either strings (= atoms) or lists. The lists can
recursively contain further S-expressions or be empty, and must be balanced,
i.e. parentheses must match.
4.3 Examples
=============
<< this_is_an_atom_123'&^%! ; this is a comment
"another atom in an OCaml-string \"string in a string\" \123"
; empty list follows below
()
; a more complex example
(
(
list in a list ; comment within a list
(list in a list in a list)
42 is the answer to all questions
)
)
>>
4.4 Conversion of basic OCaml-values
=====================================
Basic OCaml-values like the unit-value, integers (in all representations),
floats, strings, and booleans are represented in S-exp syntax in the same way
as in OCaml. Strings may also appear without quotes if this does not clash
with the lexical conventions for S-expressions.
4.5 Conversion of OCaml-tuples
===============================
OCaml-tuples are simple lists of values in the same order as in the tuple.
E.g.:
<< (3.14, "foo", "bar bla", 27) <===> (3.14 foo "bar bla" 27)
>>
4.6 Conversion of OCaml-records
================================
OCaml-records are represented as lists of pairs in S-expression syntax. Each
pair consists of the name of the record field (first element), and its value
(second element). E.g.:
<< {
foo = 3;
bar = "some string";
}
<===>
(
(foo 3)
(bar "some string")
)
>>
Type specifications of records allow the use of a special type 'sexp_option'
which indicates that a record field should be optional. E.g.:
<< type t =
{
x : int option;
y : int sexp_option;
}
>>
The type 'sexp_option' is equivalent to ordinary options, but is treated
specially by the code generator. The above would lead to the following
equivalences of values and S-expressions:
<< {
x = Some 1;
y = Some 2;
}
<===>
(
(x (some 1))
(y 2)
)
>>
And:
<< {
x = None;
y = None;
}
<===>
(
(x none)
)
>>
Note how 'sexp_option' allows you to leave away record fields that should
default to 'None'. It is also unnecessary (and actually wrong) now to write
down such a value as an option, i.e. the 'some'-tag must be dropped if the
field should be defined.
There is also a type qualifier 'sexp_list'. It behaves similar to
'sexp_option', but assumes the empty list as default value.
4.7 Conversion of sum types
============================
Constant constructors in sum types are represented as strings. Constructors
with arguments are represented as lists, the first element being the
constructor name, the rest being its arguments. Constructors may also be
started in lowercase in S-expressions, but will always be converted to
uppercase when converting from OCaml-values.
For example:
<< type t = A | B of int * float * t with sexp
B (42, 3.14, B (-1, 2.72, A)) <===> (B 42 3.14 (B -1 2.72 A))
>>
The above example also demonstrates recursion in datastructures.
4.8 Conversion of variant types
================================
The conversion of polymorphic variants is almost the same as with sum types.
The notable difference is that variant constructors must always start with an
either lower- or uppercase character, matching the way it was specified in the
type definition. This is because OCaml also distinguishes between upper- and
lowercase variant constructors. Note that type specifications containing
unions of variant types are also supported by the S-expression converter.
4.9 Conversion of OCaml-lists and arrays
=========================================
OCaml-lists and arrays are straightforwardly represented as S-expression
lists.
4.10 Conversion of option types
================================
The option type is converted like ordinary polymorphic sum types, i.e.:
<< None <===> none
Some value <===> (some value)
>>
There is a deprecated version of the syntax in which values of option type
are represented as lists in S-expressions:
<< None <===> ()
Some value <===> (value)
>>
Reading of the old-style S-expression syntax for option values is only
supported if the reference 'Conv.read_old_option_format' is set to 'true'
(currently the default, which may change soon). A conversion exception is
raised otherwise. The old format will be written only if
'Conv.write_old_option_format' is true (also currently the default). Reading
of the new format is always supported.
4.11 Conversion of polymorphic values
======================================
There is nothing special about polymorphic values as long as there are
conversion functions for the type parameters. E.g.:
<< type 'a t = A | B of 'a with sexp
type foo = int t with sexp
>>
In the above case the conversion functions will behave as if 'foo' had been
defined as a monomorphic version of 't' with ''a' replaced by 'int' on the
right hand side.
If a datastructure is indeed polymorphic, and you want to convert it, you
will have to supply the conversion functions for the type parameters at
runtime. E.g. in the above example, if you wanted to convert a value of type
''a t', you would have to write something like this:
<< sexp_of_t sexp_of_a v
>>
where 'sexp_of_a', which may also be named differently in this particular
case, is a function that converts values of type ''a' to an S-expression.
Types with more than one parameter require passing conversion functions for
those parameters in the order of their appearance on the left hand side of the
type definition.
4.12 Conversion of abstract datatypes
======================================
Of course, if you want to convert an abstract datatype to an S-expression,
you will have to roll your own conversion function, which should produce
values of type 'Sexp.t' directly. If, however, you want to make use of your
abstract type within definitions of other types, make sure that you call your
conversion function appropriately: it should be in the same scope as the
typename, and must be named 'sexp_of_{typename}'.
4.13 Conversion of hashtables
==============================
Hashtables, which are abstract values in OCaml, are represented as
association lists, i.e. lists of key-value pairs, e.g.:
<< ((foo 42) (bar 3))
>>
Reading in the above S-expression as hashtable mapping strings to integers
('(string, int) Hashtbl.t') will map '"foo"' to 42 and '"bar"' to 3.
Note that the order of elements in the list may matter, because duplicates
are kept: bindings will be inserted into the hashtable in order of appearence.
Therefore, the last binding of a key will be the "visible" one, the others are
"hidden". See the OCaml-documentation on hashtables for details.
Note, too, that polymorphic equality may not hold between conversions. You
will have to use a function implementing logical equality for that purpose.
4.14 Conversion of opaque values
=================================
Opaque values are ones for which we do not want to perform conversions. This
may be, because we do not have S-expression converters for them, or because we
do not want to apply them in a particular type context, e.g. if the resulting
S-expression should be printed out but without superfluous information. To
prevent the preprocessor from generating calls to converters, simply apply the
qualifier 'sexp_opaque' as if it were a type constructor, e.g.:
<< type foo = int * stuff sexp_opaque with sexp
>>
Thus, there is no need to specify converters for type 'stuff', and if there
are any, they will not be used in this particular context. Needless to say, it
is not possible to convert such an S-expression back to the original value.
Here is an example conversion:
<< (42, some_stuff) ===> (42, <opaque>)
>>
4.15 Conversion of exceptions
==============================
S-expression converters for exceptions can be automatically registered using
the 'with sexp' macro, e.g.:
<< module M = struct
exception Foo of int with sexp
end
>>
Such exceptions will be translated in a similar way as sum types, but their
constructor will be prefixed with the fully qualified module path (here:
'M.Foo') so as to be able to discriminate between them without problems.
The user can then easily convert an exception matching the above one to an
S-expression using 'Sexplib.Conv.sexp_of_exn'. User-defined conversion
functions can be registered, too, by calling 'Sexplib.Conv.add_exn_converter'.
This should make it very convenient for users to catch arbitrary exceptions
escaping their program and pretty-printing them, including all arguments, as
S-expressions. The library already contains mappings for all known exceptions
that can escape functions in the OCaml standard library.
5 Contact information
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In the case of bugs, feature requests and similar, you can contact us here:
mmottl@janestcapital.com
Up-to-date information concerning this library should be available here:
http://www.janestcapital.com/ocaml
Enjoy!!
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