[[LexicalChapter]]

Lexical syntax
--------------

Larceny's default lexical syntax extends the lexical syntax
required by the R5RS, R6RS, and ERR5RS <<Standards, standards>>.

The R6RS forbids most lexical extensions, however, so Larceny
provides several mechanisms for turning its lexical extensions
on and off.


[[FlagsSection]]

Flags
~~~~~

By default, Larceny recognizes several Larceny-specific flags
of the form permitted by the R6RS.  The flag you are most
likely to encounter represents one of Larceny's unspecified
values:

----------------------------------------------------------------
    #!unspecified
----------------------------------------------------------------

Certain other flags have special meanings to Larceny's
+read+ and +get-datum+ procedures.  They are described below.


[[CaseFoldingSection]]

Case-sensitivity
~~~~~~~~~~~~~~~~

By default, Larceny is case-sensitive.
This global default can be overridden by specifying
+--foldcase+ or +--nofoldcase+ on Larceny's command line,
or by changing the value of Larceny's +case-sensitive?+ parameter.

The case-sensitivity of a particular textual input port
is affected by reading one of the following flags from
the port using the +read+ or +get-datum+ procedures:

----------------------------------------------------------------
    #!fold-case
    #!no-fold-case
----------------------------------------------------------------

The +\#!fold-case+ flag enables case-folding on data read from
the port by the +read+ and +get-datum+ procedures, while the
+\#!no-fold-case+ flag disables case-folding.  The behavior
established by one of these flags extends to the next flag
read from the port by +read+ or +get-datum+.

Both +\#!fold-case+ and +\#!no-fold-case+ evaluate to an
unspecified value.  To obtain the effect of one of these flags
while treating it as a comment, place +#;+ in front of the flag.


[[LexicalExtensionsSection]]

Lexical extensions
~~~~~~~~~~~~~~~~~~

When a port is first opened, the Larceny-specific lexical
extensions that are accepted on the port are determined
by Larceny's <<LexicalParametersSection,lexical parameters>>.

The following flags change the case-sensitivity and lexical
extensions on the specific port from which they are read:

----------------------------------------------------------------
    #!r6rs         ; implies #!no-fold-case, negates other flags
    #!r5rs         ; implies #!fold-case, #!err5rs
    #!err5rs       ; allows Larceny-specific extensions
    #!larceny      ; implies #!no-fold-case, #!err5rs
----------------------------------------------------------------

The +\#!r6rs+ flag is a comment, while all of Larceny's other
flags evaluate to an unspecified value.  To obtain the effect
of a flag other than +\#!r6rs+ while treating it as a comment,
place +\#;+ in front of the flag.

[WARNING]
================================================================
The R6RS requires implementations to treat +\#!r6rs+ as a
comment; it is the only flag that implementations of the R6RS
are required to treat as a comment.  Since the +\#!r6rs+ flag
behaves differently from all other flags, it is deprecated.
================================================================


[[LexicalParametersSection]]

Lexical parameters
~~~~~~~~~~~~~~~~~~

When given no argument, these parameters return the current
default for some aspect of the lexical syntax that will be
accepted on newly created ports.
When given an argument, these procedures change the default
as specified by the argument.

proc:case-sensitive?[args="",result="boolean"]
proctempl:case-sensitive?[args="boolean"]

Determines whether newly created textual ports default to
case-sensitive.

proc:read-larceny-weirdness?[args="",result="boolean"]
proctempl:read-larceny-weirdness?[args="boolean"]

Determines whether newly created textual ports allow
Larceny's usual extensions to R5RS lexical syntax.
This parameter also determines whether newly created
ports allow +\#+ as an insignificant digit; this is
required by the R5RS, but disallowed by the R6RS.

proc:read-traditional-weirdness?[args="",result="boolean"]
proctempl:read-traditional-weirdness?[args="boolean"]

Determines whether newly created textual ports allow
certain lexical extensions that are deprecated in Larceny.
These extensions include symbols enclosed by vertical
bars and read-time evaluation.

For the current semantics of these parameters, please consult
the Larceny developers' web page that describes
https://github.com/larcenists/larceny/wiki/LexicalConversion[Larceny's
lexical syntax].