DSSSL, the Document Style Semantics and Specification Language, is a language for specifying stylesheets for SGML documents, and is semantically an ancestor of XSLT. It includes as part of itself something called the expression language, a large subset (with extensions) of R4RS Scheme. This page tells you in a coarse way (see the previous link for details) what the differences are between the two.
Here's the list provided at the top of Clause 8:
- The expression language uses only the functional, side-effect free subset of Scheme. Features of Scheme that are not useful in the absence of side-effects have been removed (for example, begin, set!, eqv?)
- The vector data type is not provided.
- A character object is uniquely identified by its name and properties.
- The number data type is a subtype of a more general quantity data type that adds the concept of dimension to a number.
- call-with-current-continuation is not provided.
- Some optional features of R4RS are not provided.
- The gcd and lcm procedures are not provided.
- Keyword arguments are provided.
The syntax keywords and procedures provided by the expression language and its defined subset, the core expression language, are enumerated here.
Here are finer details:
- The list of syntactic keywords is not extensible.
- You may not redefine a defined identifier.
- Improper lambda-lists are not allowed.
- A definition may refer to identifiers that are defined in later definitions, not only within lambda-expressions, but anywhere.
- Internal definitions are equivalent to letrec.
- You may redefine a built-in identifier such as car, and such redefinitions are pervasive.
- Case uses equal? to match keys, since eqv? does not exist.
- Numbers are restricted to exact integers and inexact rationals.
- Exact integers must include the range -231-1 to 231-1 and may be larger.
- #i and #e syntax are not supported.
- Complex lambda-lists are provided, with flags #!optional, #!rest, #!key. These three are self-defining named constants when used outside lambda-lists.
- Keywords are runtime objects, with procedures keyword?, keyword->string, string->keyword.
- Quantities are a superset of numbers: the primitive unit is meters, written m (e.g. 1m for 1 meter). Standard derived units are cm mm in pt pica, and new derived units (but not new primitive units) may be added with define-unit.
- Numeric literals with a decimal point, an exponent, or a unit are inexact; all others are exact.
- The procedures exact?, inexact?, =, <, >, <=, >=, zero?, positive?, negative?, odd?, even?, max, min, +, -, *, /, abs atan (with two arguments), sqrt apply to quantities as well as numbers.
- quantity->number strips the unit from a quantity.
- format-number converts a number to a string containing ordinary numbers, numbers with leading zeros, letters, or Roman numerals. format-number-list applies to a
- Characters have (immutable) property lists, where the properties are keywords: one property is numeric-equiv:.
- Language objects specify collation and case conversion; procedures are language?, current-language, with-language.
- Non-Scheme procedures can be accessed with external-procedure, which returns a corresponding Scheme procedure, which should be pure functional.
- Date and time procedures are provided: time returns the current time since the 1970 Epoch as an integer; time->string, time<?, time>?, time<=?, time>= generate and compare ISO 8601 timestamp strings.
- error signals an error; it has one argument, a string.