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39 lines
3.7 KiB
TeX
39 lines
3.7 KiB
TeX
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\chapterprecishere{``Caution! Under no circumstances confuse the adjective \emph{basic} with the noun \emph{BASIC}, except under confusing circumstances!''\par\raggedleft --- \textup{\tbas\ Reference Manual, \theedition}\footnote{Original quotation: Donald R. Woods, James M. Lyon, \emph{The INTERCAL Programming Language Reference Manual}}}
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This chapter describes the basic concepts of the \tbas\ language.
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\section{Values and Types}
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\label{valuesandtypes}
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BASIC is a \emph{Dynamically Typed Language}, which means variables do not know which group they should barge in; only values of the variable do. In fact, there is no type definition in the language: \emph{we do want our variables to feel awkward.}
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There are six basic types: \emph{number}, \emph{boolean}, \emph{string}, \emph{array}, \emph{generator} and \emph{function}.
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\emph{Number} represents real (double-precision floating-point or actually, \emph{rational}) numbers. Operations on numbers follow the same rules of the underlying virtual machine\footnote{if you are not a computer person, disregard}, and such machines must follow the IEEE 754 standard\footnote{ditto.}.
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\emph{Boolean} is type of the value that is either \codebf{TRUE} or \codebf{FALSE}. Number \codebf{0} and \codebf{FALSE} make the condition \emph{false}. When used in numeric context, \codebf{FALSE} will be interpreted as 0 and \codebf{TRUE} as 1.
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\emph{String} represents immutable\footnote{Cannot be altered directly} sequences of bytes. However, you can't weave them to make something like \emph{string array}\footnote{Future feature\ldots\ maybe\ldots? Probably not\ldots}.
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\emph{Array} represents a collection of numbers in one or more dimensions.
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\emph{Generator} represents a value that automatically counts up/down whenever they have been called in FOR--NEXT loop.
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\emph{Functions} are, well\ldots\ functions\footnote{This is not a closure; there is no way you can define a local- or anonymous variable in BASIC.}, especially user-defined ones. Functions are \emph{type} because some built-in functions will actually take \emph{functions} as arguments.
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\section{Control Flow}
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A program is executed starting with its lowest line number. Statements on a line are executed from left to right. When all Statements are finished execution, next lowest line number will be executed. Control flow functions can modify this normal flow.
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You can dive into other lines in the middle of the program with \code{GOTO}. The program flow will continue normally at the new line \emph{and it will never know ya just did that}.
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If you want less insane jumping, \code{GOSUB} is used to jump to a subroutine. Subroutine is a little section of a code that serves as a tiny program inside of a program. \code{GOSUB} will remember from which statement in the line you have came from, and will return your program flow to that line when \code{RETURN} statement is encountered. (of course, if \code{RETURN} is used without \code{GOSUB}, program will raise some error) Do note that while you can reserve some portion of a program line as a \code{subroutine}, \tbas\ will not provide local variables and whatnot as all variables in BASIC are global, and you can just \code{GOTO} out of a subroutine to anywhere you desire and wreak havoc \emph{if you really wanted to}.
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The \code{ON} statement provides alternative branching construct. You can enter multiple line numbers and let your variable (or mathematical expression) to choose which index of line numbers to \code{GOTO}- or \code{GOSUB} into.
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The \code{IF-THEN-ELSE} lets you to conditionally select which of the two branches to execute.
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The \code{FOR-NEXT} lets you to loop a portion of a program while automatically counting your chosen variable up or down.
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