Icon (programming language)


Icon is a very high-level programming language featuring goal-directed execution and many facilities for managing strings and textual patterns. It is related to SNOBOL and SL5, string processing languages. Icon is not object-oriented, but an object-oriented extension called Idol was developed in 1996 which eventually became Unicon.

Basic syntax

The Icon language is derived from the ALGOL-class of structured programming languages, and thus has syntax similar to C or Pascal. Icon is most similar to Pascal, using syntax for assignments, the keyword and similar syntax. On the other hand, Icon uses C-style brackets for structuring execution groups, and programs start by running a procedure called.
In many ways Icon also shares features with most scripting languages : variables do not have to be declared, types are cast automatically, and numbers can be converted to strings and back automatically. Another feature common to many scripting languages, but not all, is the lack of a line-ending character; in Icon, lines not ended by a semicolon get ended by an implied semicolon if it makes sense.
Procedures are the basic building blocks of Icon programs. Although they use Pascal naming, they work more like C functions and can return values; there is no keyword in Icon.

procedure doSomething
write
end

Goal-directed execution

One of Icon's key concepts is that control structures are based on the "success" or "failure" of expressions, rather than on boolean logic, as in most other programming languages. This feature derives directly from SNOBOL, in which any pattern match and/or replacement operation could be followed by success and/or failure clauses that specified a statement label to be branched to under the requisite condition. Under the goal-directed branching model, a simple comparison like does not mean, "if the operations to the right evaluate to true" as they would under most languages; instead, it means something more like, "if the operations to the right succeed". In this case the < operator succeeds if the comparison is true, so the end result is the same. In addition, the < operator returns its second argument if it succeeds, allowing things like, a common type of comparison that in most languages must be written as a conjunction of two inequalities like.
Icon uses success or failure for all flow control, so this simple code:

if a := read then write
will copy one line of standard input to standard output. It will work even if the read causes an error, for instance, if the file does not exist. In that case the statement a := read will fail, and write will simply not be called.
Success and failure are passed "up" through functions, meaning that a failure inside a nested function will cause the functions calling it to fail as well. For instance, here is a program that copies an entire file:

while write)
When the read command fails, at the end of file for instance, the failure will be passed up the call chain, and write will fail as well. The while, being a control structure, stops on failure. A similar example written in pseudocode :

try catch

This case needs two comparisons: one for end of file and another for all other errors. Since Java does not allow exceptions to be compared as logic elements, as under Icon, the lengthy syntax must be used instead. Try blocks also impose a performance penalty, even if no exception is thrown, a distributed cost that Icon avoids.
Icon refers to this concept as goal-directed execution, referring to the way that execution continues until some goal is reached. In the example above the goal is to read the entire file; the read command succeeds when information has been read, and fails when it hasn't. The goal is thus coded directly in the language, instead of by checking return codes or similar constructs.

Generators

Expressions in Icon often return a single value, for instance, will evaluate and succeed if the value of x is less than 5, or else fail. However, many expressions do not immediately return success or failure, returning values in the meantime. This drives the examples with and ; causes to continue to return values until it fails.
This is a key concept in Icon, known as generators. Generators drive much of the loop functionality in the language, but without the need for an explicit loop comparing values at each iteration.
Within the parlance of Icon, the evaluation of an expression or function produces a result sequence. A result sequence contains all the possible values that can be generated by the expression or function. When the result sequence is exhausted the expression or function fails. Iteration over the result sequence is achieved either implicitly via Icon's goal-directed evaluation or explicitly via the clause.
Icon includes several generator-builders. The alternator syntax allows a series of items to be generated in sequence until one fails:

1 | "hello" | x < 5
can generate "1", "hello", and "5" if x is less than 5. Alternators can be read as "or" in many cases, for instance:

if y < then write
will write out the value of y if it is smaller than x or 5. Internally Icon checks every value from left to right until one succeeds or the list empties and it returns a failure. Functions will not be called unless evaluating their parameters succeeds, so this example can be shortened to:

write
Another simple generator is, which generates lists of integers; will call ten times. The bang syntax generates every item of a list; will output each character of aString on a new line.
This concept is powerful for string operations. Most languages include a function known as or that returns the location of one string within another. For example:

s = "All the world's a stage. And all the men and women merely players";
i = indexOf
This code will return 4, the position of the first occurrence of the word "the". To get the next instance of "the" an alternate form must be used,

i = indexOf
the 5 at the end saying it should look from position 5 on. So to extract all the occurrences of "the", a loop must be used:

s = "All the world's a stage. And all the men and women merely players";
i = indexOf
while i != -1
Under Icon the function is a generator, and will return the next instance of the string each time it is resumed before failing when it reaches the end of the string. The same code can be written:

s := "All the world's a stage. And all the men and women merely players"
every write
will return the index of the next instance of "the" each time it is resumed by, eventually reaching the end of the string and failing.
Of course there are times where one wants to find a string after some point in input, for instance, if scanning a text file containing data in multiple columns. Goal-directed execution works here as well:

write

The position will only be returned if "the" appears after position 5; the comparison will fail otherwise. Comparisons that succeed return the right-hand result, so it is important to put the find on the right-hand side of the comparison. If it were written:

write

then "5" would be written instead of the result of.
Icon adds several control structures for looping through generators. The operator is similar to, looping through every item returned by a generator and exiting on failure:

every k := i to j do
write
re-evaluates the first result, whereas produces all results.
The syntax actually injects values into the function in a fashion similar to blocks under Smalltalk. For instance, the above loop can be re-written this way:

every write

Generators can be defined as procedures using the keyword:

procedure findOnlyOdd
every i := find do
if i % 2 = 1 then suspend i
end
This example loops over theString using find to look for pattern. When one is found, and the position is odd, the location is returned from the function with. Unlike, memorizes the state of the generator, allowing it to pick up where it left off on the next iteration.

Strings

Icon has features to make working with strings easier. Most notable is the scanning system, which repeatedly calls functions on a string:
s ? write
is a short form of the examples shown earlier. In this case the subject of the function is placed outside the parameters in front of the question mark. Icon function signatures identify the subject parameter so that it can be hoist in this fashion.
Substrings can be extracted from a string by using a range specification within brackets. A range specification can return a point to a single character, or a slice of the string. Strings can be indexed from either the right or the left. Positions within a string are defined to be between the characters 1A2B3C4 and can be specified from the right −3A−2B−1C0
For example,

"Wikipedia" > "W"
"Wikipedia" > "k"
"Wikipedia" > "a"
"Wikipedia" > "Wi"
"Wikipedia" > "ia"
"Wikipedia" > "iki"

Where the last example shows using a length instead of an ending position
The subscripting specification can be used as a lvalue within an expression. This can be used to insert strings into another string or delete parts of a string. For example,

s := "abc"
s := "123"
s now has a value of "a123c"
s := "abcdefg"
s := "ABCD"
s now has a value of "abABCDefg"
s := "abcdefg"
s := ""
s now has a value of "abefg"

Icon's subscript indices are between the elements. Given the string s := "ABCDEFG", the indexes are: 1A2B3C4D5E6F7G8. The slice s is the string between the indices 3 and 5, which is the string "CD".

Other structures

Icon also has syntax to build lists :
aCat :=
The items within a list can be of any type, including other structures. To build larger lists, Icon includes the generator; generates a list containing 10 copies of "word".
Like arrays in other languages, Icon allows items to be looked up by position, e.g.,. As with strings, the indices are between the elements, and a slice of a list can be obtained by specifying the range, e.g., produces the list. Unlike for strings, a slice of an array is not an lvalue.
The bang-syntax enumerates the range. For example, will print out four lines, each with one element.
Icon includes stack-like functions, and to allow arrays to form the bases of stacks and queues.
Icon also includes functionality for sets and associative arrays with tables:

symbols := table
symbols := 1
symbols := 2
This code creates a table that will use zero as the default value of any unknown key. It then adds two items into it, with the keys "there" and "here", and values 1 and 2.

String scanning

One of the powerful features of Icon is string scanning. The scan string operator,, saves the current string scanning environment and creates a new string scanning environment. The string scanning environment consists of two keyword variables, &subject and, where &subject is the string being scanned, and &pos is the cursor or current position within the subject string.
For example,

s := "this is a string"
s ? write

would produce

subject= pos=

Built-in and user-defined functions can be used to move around within the string being scanned. Many of the built-in functions will default to &subject and &pos. The following, for example, will write all blank-delimited "words" in a string.

s := "this is a string"
s ?

A more complex example demonstrates the integration of generators and string scanning within the language.

procedure main
s := "Mon Dec 8"
s ? write
end
# Define a matching function that returns
# a string that matches a day month dayofmonth
procedure Mdate
# Define some initial values
static dates
static days
initial
every suspend # Followed by at least 2 digits
) &
& # Either a blank or the end of the string
retval # And finally return the string
end
# Matching function that returns a string of n digits
procedure matchdigits
suspend & v
end

The idiom of returns the value of the last expression.