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One's Nightmare is Another's Demo

BabyCobol is a language that was designed to be the “worst nightmare” of compiler writers, deliberately combining worst features of the oldest and the ugliest programming languages (YT). Unfortunately, some of the most important software — that of banks, insurance and booking companies, military — is written in such languages, and in order to tackle them successfully, you need to train on smaller prey. BabyCobol is used in one of the CS MSc courses at UT, Software Evolution, where students treat it as a challenge and work towards its full implementation over the two months. For them, the goal is to implement as many features as possible, no matter the cost.

The goal of this project is different: take one language framework (good lists can be found on this website under "Technologies" tab or in this paper in §4), implement whatever you can in it and report on difficulties. Sounds easy, right? Well, perhaps all this talk about BabyCobol being a nightmare is just for show…

Setup Instructions

  1. This software requires Apache Maven in order to function.
  2. With mvn package the tests are executed and a JAR file will be packaged.
  3. This JAR can then be used to run the parser over source files.

Thesis

The final thesis itself be found on essay.utwente.nl

BabyCobol Language

Beneath one will find a quick and very brief language reference of the BabyCobol language.

Syntax

Position-Based Syntax

Column(s) Description
1 - 6 Sequence number
7 Line status indicator
8 Start top-level constructs
12 - 72 Regular code
73+ Ignored

Line Continuation

Since the language is designed to be fitted on old-fashioned punch cards, it might be the case that lines are longer than that they can physically fit on a given card. To indicate that a line is continued, the next line contains a marker, a dash, in the 7th column. Indicating that it is a continuation from the previous line.

Case Sensitivity

BabyCobol is case-insensitive. However, there is one edge-case. Keywords should always be uppercase.

Non-reserved Keywords

Keywords are not reserved in the language and can also be used as variables, this would make the following statement completely valid:

␣␣␣␣␣␣␣␣␣␣05 END PICTURE IS 99.

Even though END is a reserved keyword.

White Space

White spaces are ignored, but spaces are required around the - operator when used as an infix arithmetic operator.

Lexical Imports

BabyCobol has support for verbatim imports. With the COPY directive these are instantly put in the file, if a compiler error occurs, then these are shown inside the file itself and not the location into which it was copied.

Additionally, a REPLACE directive allows to replace a string while copying a code file.

Code Structure

A program consists out of divisions, which each consist out of sections. In total there are three divisions:

  1. The IDENTIFICATION_DIVISION
    1. Contains basic program information, no real required names and values are however required.
  2. The DATA_DIVISION
    1. Narrowed down to two basic data types
  3. The PROCEDURE_DIVISION
    1. Mostly identical to COBOL.
    2. Consists out of arbitrarily named sections, which have paragraphs. Both sections and paragraphs are named and callable. A paragraph consists out of sentences which in turn has statements.

Statement separators

Statements do not have to be, but can be separated by a period (.).

Jumps

With the directive NEXT SENTENCE one jumps immediately to the next sentence.

Semantics

Data Types

There are three data types: picture clauses, occurs clauses and like clauses.

Picture Clause

Picture clauses are primitive type definitions which revolve around a pattern for storage and display value. For example, the field VAR is defined as VAR PICTURE IS $999V99 will have five decimal places, with each time printed a dollar sign as first symbol, 3 digits either a comma or period (depending on locale) and 2 more digits. The following special characters are used in BabyCobol for such formatting:

  • 9 - Any digit
  • A - An alphabet character or a space
  • X - Any single character
  • Z - A leading digit, disappearing into space if zero
  • S - A sign (+ or -, space is treated as a plus)
  • V - A decimal separator (usually . or ,)

Any other characters are used verbatim. Any number of consequenent characters is can be replaced as C(N) where C is the character and N the amount of repetitions. Hence 999999V99 is equal to 9(6)V9(2), and X(20) is any string that is 20 characters long.

Occurs Clause

Occurs clauses are used to define arrays, the code fragment below creates an array with length 20. And puts a single picture element in it. To reference the element back, one would have to use ELEMENT OF ARRAY(1).

01 ARRAY OCCURS 20.
   03 ELEMENT PICTURE IS 9(15)V99.

Like Clause

Like clauses allows to reuse the type of a variable that was already defined. In the example 03 NEW-FIELD LIKE ANOTHER-FIELD. The NEW-FIELD variable will reuse the type of ANOTHER-FIELD. A note, OCCURS is not treated as an essential part of the type, so using LIKE on OCCURS

Arithmetic Operations

Arithmetic operations may only be performed on atomic fields, which pictures are free of any characters (so no X or A) fields. The same can be said for infix operators, expect for +, where it can be used to append strings. Comparisons work symbol by symbol, space-padding the shortest of the values on the left.

Qualification

Qualified variable names can grow long, hence it is possible to only communicate a part of the path towards the field, the rest is automatically inferred. If a datastructure would exist where the field FOO has a field BAR which has a field F, then the full name would be F OF BAR OF FOO. If there wouldn't be any other competing Fs then just the F could be enough. Or F OF FOO, where BAR is inferred. Or F OF BAR where the root is inferred.

If ambiguity arises, the type and context information will be used. If still ambiguity remains a proper error message is provided.

Figurative Constants

BabyCobol implements three figurative constants: SPACES, LOW-VALUES and HIGH-VALUES. Where MOVE SPACES X would set all spaces in the X variable, and for every inner layer if this is a compound data structure. LOW-VALUES puts everything in the lowest possible value, so X PICTURE IS 99, would become 0. But X PICTURE IS S999 would be -999.

Default Values

If a unknown variable is used, then it will be instantiated with its own upper-case name. So ACCOUNT-NAME will become "ACCOUNT-NAME". Where the inferred data type is PICTURE IS X(N), where N is the length of the variable name.

Conditional Contractions

One can write IF X = 2 OR 4, instead of IF X = 2 or X = 4. Operator priorities work normally so IF X = 42 OR 3 AND Y < X is equal to IF X = 42 OR (3 AND Y < X).

Semantics of Statements

Unconditional go to

The GO TO statement unconditionally jumps to a different procedure. The ALTER statement will update a GO TO, to jump to another location.

Perform

PERFORM calls a procedure and returns to continue execution on the next statement. It supports a way to call several subsequently written paragraphs as one.

Loop

LOOP will execute a single statement multiple times, and can act as a while, for or do-while loop. The clauses for this loop do not have to occur at the start, but can appear at any point in the body.

Name-Driven Assignment

MOVE A TO B will move through all the fields of A and map all the values from A to the name-matching fields in B discarding the rest. Move is also used in Figurative Constants.

Exception Handling

The SIGNAL keyword allows for one error handler and catches all runtime errors.