1 H-expressions
The Scheme reader incorporates an H-expression reader, and Scheme’s printer also supports printing values in Honu syntax. The reader can be put into H-expression mode either by including #hx in the input stream, or by calling read-honu or read-honu-syntax instead of read or read-syntax. Similarly, print (or, more precisely, the default print handler) produces Honu output when the print-honu parameter is set to #t.
When the reader encounters #hx, it reads a single H-expression, and it produces an S-expression that encodes the H-expression. Except for atomic H-expressions, evaluating this S-expression as Scheme is unlikely to succeed. In other words, H-expressions are not intended as a replacement for S-expressions to represent Scheme code.
Honu syntax is normally used via #lang honu, which reads H-expressions repeatedly until an end-of-file is encountered, and processes the result as a module in the Honu language.
Ignoring whitespace, an H-expression is either
a number (see Numbers);
an identifier (see Identifiers);
a string (see Strings);
a character (see Characters);
a sequence of H-expressions between parentheses (see Parentheses, Brackets, and Braces);
a sequence of H-expressions between square brackets (see Parentheses, Brackets, and Braces);
a sequence of H-expressions between curly braces (see Parentheses, Brackets, and Braces);
a comment followed by an H-expression (see Comments);
#; followed by two H-expressions (see Comments);
#hx followed by an H-expression;
#sx followed by an S-expression (see The Reader).
Within a sequence of H-expressions, a sub-sequence between angle brackets is represented specially (see Parentheses, Brackets, and Braces).
Whitespace for H-expressions is as in Scheme: any character for which char-whitespace? returns true counts as a whitespace.
1.1 Numbers
The syntax for Honu numbers is the same as for Java. The S-expression encoding of a particular H-expression number is the obvious Scheme number.
1.2 Identifiers
The syntax for Honu identifiers is the union of Java identifiers plus ;, ,, and a set of operator identifiers. An operator identifier is any combination of the following characters:
+ - _ = ? : < > . ! % ^ & * / ~ |
The S-expression encoding of an H-expression identifier is the obvious Scheme symbol.
Input is parsed to form maximally long identifiers. For example, the input int->int; is parsed as four H-expressions represented by symbols: 'int, '->, 'int, and '|;|.
1.3 Strings
The syntax for an H-expression string is exactly the same as for an S-expression string, and an H-expression string is represented by the obvious Scheme string.
1.4 Characters
The syntax for an H-expression character is the same as for an H-expression string that has a single content character, except that a ' surrounds the character instead of ". The S-expression representation of an H-expression character is the obvious Scheme character.
1.5 Parentheses, Brackets, and Braces
A H-expression between ( and ), [ and ], or { and } is represented by a Scheme list. The first element of the list is '#%parens for a (...) sequence, '#%brackets for a [...] sequence, or '#%braces for a {...} sequence. The remaining elements are the Scheme representations for the grouped H-expressions in order.
In an H-expression sequence, when a < is followed by a >, and when nothing between the < and > is an immediate symbol containing a =, &, or |, then the sub-sequence is represented by a Scheme list that starts with '#%angles and continues with the elements of the sub-sequence between the < and > (exclusive). This representation is applied recursively, so that angle brackets can be nested.
An angle-bracketed sequence by itself is not a single H-expression, since the < by itself is a single H-expression; the angle-bracket conversion is performed only when representing sequences of H-expressions.
Symbols with a =, &, or | prevent angle-bracket formation because they correspond to operators that normally have lower or equal precedence compared to less-than and greater-than operators.
1.6 Comments
An H-expression comment starts with either // or /*. In the former case, the comment runs until a linefeed or return. In the second case, the comment runs until */, but /*...*/ comments can be nested. Comments are treated like whitespace.
A #; starts an H-expression comment, as in S-expressions. It is followed by an H-expression to be treated as whitespace. Note that #; is equivalent to #sx#;#hx.
1.7 Honu Output Printing
Some Scheme values have a standard H-expression representation. For values with no H-expression representation but with a readable S-expression form, the Scheme printer produces an S-expression prefixed with #sx. For values with neither an H-expression form nor a readable S-expression form, then printer produces output of the form #<...>, as in Scheme mode. The print-honu parameter controls whether Scheme’s printer produces Scheme or Honu output.
The values with H-expression forms are as follows:
Every real number has an H-expression form, although the representation for an exact, non-integer rational number is actually three H-expressions, where the middle H-expression is /.
Every character string is represented the same in H-expression form as its S-expression form.
Every character is represented like a single-character string, but (1) using a ' as the delimiter instead of ", and (2) protecting a ' character content with a \ instead of protecting " character content.
A list is represented with the H-expression sequence list(⟨v⟩, ...), where each ⟨v⟩ is the representation of each element of the list.
A pair that is not a list is represented with the H-expression sequence cons(⟨v1⟩, ⟨v2⟩), where ⟨v1⟩ and ⟨v2⟩ are the representations of the pair elements.
A vector’s representation depends on the value of the print-vector-length parameter. If it is #f, the vector is represented with the H-expression sequence vectorN(⟨v⟩, ...), where each ⟨v⟩ is the representation of each element of the vector. If print-vector-length is set to #t, the vector is represented with the H-expression sequence vectorN(⟨n⟩, ⟨v⟩, ...), where ⟨n⟩ is the length of the vector and each ⟨v⟩ is the representation of each element of the vector, and multiple instances of the same value at the end of the vector are represented by a single ⟨v⟩.
The empty list is represented as the H-expression null.
True is represented as the H-expression true.
False is represented as the H-expression false.