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DSSSLVsScheme.md

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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.

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 -2^31^-1 to 2^31^-1 and may be larger.
  • #i and #e syntax are not supported.

DSSSL extensions:

  • 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. A keyword is notated foo:.
  • 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 list of numbers.
  • 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.