Sketchy LISP
Reference
  Copyright (C) 2006
Nils M Holm

2 Programs

2.1 Program Execution

Each Sketchy expression is a program and each program is an expression. Programs are executed by reducing them to their normal forms. An expression is in its normal form, if it cannot be reduced any further. The interpreter prints the result of each expression that has been read at the top level. The context of an expression is the top level, if the expression has been entered at the user's terminal (or redirected from a file).

The interpreter makes no difference between upper and lower case characters, but it folds all input to lower case.

2.2 Expressions

There are multiple types of Sketchy expressions:

The notation () represents the empty list.

Symbols, booleans, procedures, numbers, chars, and strings are called atoms or atomic expressions, because they cannot be decomposed.

A symbol is represented by a name composed of these ASCII characters:

abcdefghijklmnopqrstuvwxyz
0123456789
* + - / < = > _ ? !

A pair is used to glue two expressions together. Is is represented by the form

(x . y)

where x and y may be of any type. The x part of a pair is also called its car part and the y part is called its cdr part part. A pair is also said to be of the type cons.

Note that the Scheme standard requires a blank character before and after the dot separating the car and cdr part of a pair. Sketchy does not require this blank, so above pair could be written as:

(x.y)

A list is a pair whose cdr field is either another pair or (). The only exception is the empty list which does not have a car nor a cdr part. Each list of the form

(a1 . (a2 . ... (aN . ()) ... ))

may also be written as

(a1 a2 ... aN)

The second form is equivalent to the first one, but easier to read.

Lists whose last members are not equal to () are called improper lists. Such lists have an atom in their last position. They are represented using pair notation:

(a . b)

or

(a b . c)

The boolean literal #t represents logical truth and #f represents logical falsity. Note that #t merely is the canonical form of the true value. Any object that is not identical to #f is considered true.

Integer numbers (or just integers) are represented by lists of digits with an optional leading plus or minus sign, eg:

314 -159 +265

Integers are the only numbers in Sketchy, so the terms integer and number are used as synonyms in this document.

Character literals (or just chars) are introduced by the prefix #\. The character to be represented follows without any blanks in between, for instance:

#\A  #\b  #\"  #\\  #\#  #\(

The blank character may be written as #\ , but for improved readability, the notation #\space should be preferred.

String literals (or just strings) are sequences of printable characters (including blanks) which are delimited by double quotes ("):

"abc"  "Foo"  "Hello, World!"  "\"Hi,\" she said."

As shown above, quote characters may be included in strings by escaping them. A character is escaped by prefixing it with a backslash (\). To include the backslash itself in a string, it must be escaped with another backslash:

"A \\ B"

Procedures are represented by lambda functions of the form

(lambda (args) term)

They will be explained in detail in the following chapters.

Any expression that is prefixed with an apostrophe already is in its normal form. Hence

(+ 5 7) => 12

but

'(+ 5 7) => (+ 5 7)

At any point, a comment may be placed in an expression by inserting a semicolon (;). Each comment extends up to the end of the line it has been started in. All characters are allowed in comments. To the interpreter, the entire comment looks like a single space character.

2.3 External Representation

The readable notation of a Sketchy (or Scheme) object is called its external representation. For example, 123 is the external representation of an integer with the value 123, "Hello, World!" is the external representation of a string with the content Hello, World!.

The external representation of an object is the form that is used to store that object externally (i.e. outside of the interpreter). The write procedure writes the external representation of a given expression to the output stream. The read procedures reads the external representation of an expression and stores it internally.

Most objects that can be written using write have an unambiguous external representation. When writing such an object and then re-reading it, the original object and the object read are guaranteed to be equal (but not identical). Objects that do not have unambiguous external representations cannot be re-read and hence cause an error when a program attempts to read them.

Objects without an unambiguous external representation include procedures, continuations, the EOF object, and the void object. All these objects are represented using the following notation

#<some informative text>

Here is a summary of all Sketchy objects including examples of their external representations:

foo                symbol
"some text"        string
123                integer
#\y                char
(foo . bar)        pair
(foo bar baz)      list
#t                 boolean
#<closure (x)>     closure
#<primitive car>   primitive procedure
#<special lambda>  special form
#<continuation>    continuation
#<eof>             end-of-file object
#<void>            void object