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Text File | 1986-04-26 | 19KB | 401 lines

A Quick Look At Fifth - ----- ---- -- ----- -- The Philosophy of Fifth -- Fifth is first and foremost a programmer's programming environment. Fifth is made up of a compiler, a text editor, a dictionary editor, and an interactive environment. These elements are highly integrated to produce the Fifth programming environment. Fifth as a language is very simple and very flexible. Fifth does not attempt to provide every structure a programmer might want. Instead, Fifth gives the programmer the tools to create any structure he/she needs at compile time, easily and efficiently. The main goal of Fifth is to eliminate the long edit-compile- debug times typical of compiled languages. This cycle is reduced in several ways: 1) Incremental Compilation. Fifth limits compiles to sections of code which have been changed, as they are changed, rather than requiring a recompilation of the entire program. 2) Efficient Use of Primary Memory. The entire Fifth system is very small and thus can remain in primary memory at all times. (Any portion of the system that is not required may be removed by the programmer to further reduce size.) 3) Fast Compilation. The Fifth compiler is extremely fast; Fifth code compiles an order of magnitude faster than conventional compilers. 4) Minimal Bookkeeping. The basis behind Fifth is very simple and natural. The programmer spends very little time keeping books. -- Programming in Fifth -- Fifth has an extremely simple syntax. There are only two distinct syntactic constructs in Fifth: modules and numbers. modules and numbers must be separated by whitespace (blanks, tabs, carriage returns, linefeeds, or nulls). Thus the only syntax error possible is that of an invalid module or number. Programs are written in Fifth by writing modules using the primitive modules (the modules that come with the system) and other user defined modules. All modules are kept in a tree structured dictionary which defines the scoping of the modules. Both modules and the dictionary can be easily modified; Modules are altered using the module editor, and the dictionary using the dictionary editor. But what is a module? A Fifth module is like a subroutine or a procedure in other languages. But it is more. A module is made up of four parts: 1) The module name, which is used to reference the module. A name is made up of any combination of characters other than whitespace (Carriage returns, line feeds, nulls, blanks, and tabs). 2) The source code which defines the module, 3) The executable code which the compiler produces from the source code, 4) And a subdictionary which may contain submodules used in the module's definition. Numbers are combinations of digits and characters such that the same combination does not exist in the dictionary, and all characters are less than the current radix. There are two types of numbers, integers and reals. If the number contains a decimal or exponent, it's real; otherwise it's integer. Single precision numbers are in the range -2,147,483,648 to 2,147,483,647 and are 4 bytes long. The real numbers are also 32 bits. The precision is approximately 7 significant digits, and the exponent range is -38 to +37. -- The Dictionary -- Fifth's dictionary is an ideal data structure for programs. Both user defined modules and primitive modules (modules that come with the system) are contained in this structure. The source or text code for all modules is also kept in the dictionary. The scoping of a program is expressed directly in the dictionary, and can be easily examined and/or changed by using the dictionary editor. Thus what modules are accessible by a module is under programmer control through the Fifth programming environment. The Fifth dictionary is hierarchical; every module owns a subdictionary and belongs to another subdictionary, with the exception of the root module which owns the highest subdictionary. The owner of a particular subdictionary is called the local module of that subdictionary. The local module is the only module that has access its subdictionary. Thus a module in Fifth can be broken down into smaller logical parts, tied together by a main module, with all the parts in placed in the main module's subdictionary. When the main module is moved about in the dictionary, the subdictionary and all modules within the subdictionary are moved too. Thus programs in Fifth are written by building up modules from the primitive modules. Program development becomes a process of writing modules, testing them, moving them about in the dictionary, and using the new modules to write other modules. Thus elements of a program (modules) are modified using the module editor. The structure of the program is edited using the dictionary editor. This division is very natural, and the Fifth programming environment makes it easy. -- Scoping Rules -- In general, scoping is a term used to describe what what programming structures can or can not be accessed from any position in a program. Many languages use global scoping, which means that all subprograms or routines can access all other subprograms or routines. In large programs (and even small programs) this can lead to problems. Fifth uses scoping rules that are similar to Pascal's. In Fifth, scoping is defined by a module's position in the dictionary. This section discusses how the dictionary is used to define the scoping of a program. Each module in Fifth contains a subdictionary, which is simply a list of other modules. (This list can be empty.) The modules in the subdictionary are available only to the local module, and no other module outside the subdictionary. When a module is is used in a definition, the module's definition is needed. The Fifth system attempts to find the module by doing a name search in the dictionary. The search starts in the subdictionary of the module being defined. If the definition is not found, successively higher subdictionaries are searched, up to the root module, which is local to the highest subdictionary. If no definition is found for the module, (and the module is not a valid number) it is considered undefined. That is, the module may exist somewhere in the dictionary, but is considered undefined if it is not in the search path. FIFTH | | { A B C } | | | | | +--------------------------+ | +------------+ | | | | { D E F } { G H I } { J K } | | | | +--+ | | | | { L } { M N } { O P } Figure 1.1 For example, suppose the dictionary looked something like figure 1.1. The dictionary search starts with the lowest module accessible from the local module, and proceeds to the left, then upward, until the root module is reached. The search NEVER goes down, nor to the right. Suppose that {M} in figure 1.1 attempts to use the undefined module {X}. The modules examined while searching for {X} are, in order: {M}, {I}, {H}, {G}, {B}, {A}, and {FIFTH}. In this case M is the lowest accessible module, since {M}'s subdictionary is empty. But suppose that {K} attempted to use {A}. This time the search path is {P}, {O}, {K}, {J}, {C}, {B}, and {A}. Notice had we been searching for {M}, with {K} as the local module, {M} would be considered undefined. About the easiest way to learn the Fifth system is to jump on and start using it. Fifth places a high priority on speed, power, and flexibility, but it is very easy to use and learn. As you read through the manual, try out the examples. As quick as you can, write programs. You will find that using bit and pieces of the supplied programs quite easy and useful. -- The Dictionary Editor -- The dictionary editor allows the programmer to edit Fifth's dictionary. The following is a bit of terminology which will make explanation of the dictionary editor and its use easier. -- Terminology -- Before the dictionary is altered by the programmer, it has only the root module, and its subdictionary. The root module is originally called {FIFTH}, and the root module's subdictionary holds only the primitive modules. ( Primitive modules are all those modules basic to Fifth. Primitives differ from user defined modules in that they come with the system, and do not have subdictionaries of their own.) When the programmer brings up the Fifth system for the first time, {FIFTH}, the root module, becomes the local module. The text for the local module may be edited and compiled. Submodules may be created, deleted, renamed, and moved in the local module's subdictionary. A submodule may be inspected, in which case it becomes the local module, complete with its own subdictionary. The local module may be exited, in which case the parent module (The module "above") again becomes the local module. The local module specifies what modules are visible from the interactive environment. The interactive environment has access to a module if and only if the local module has access to that module. Thus the interactive mode is an excellent place to test modules and their interactions. As a reminder, the prompt in the interactive mode is made up of the name of the local module followed by a '>' symbol. Thus if {FIFTH} is the local module, the prompt in the interactive mode will be: FIFTH> The behavior of the interactive mode is much the same as most other FORTH systems. Any sequence of modules may be entered and executed. New modules may be defined, the local module may be redefined, etc. The dictionary editor is invoked by entering the module {DIR} form the interactive environment. It displays all the modules in the local dictionary (The subdictionary of the local module). Modules in the local dictionary may be selected with the right and left arrows. When a module is selected, it is shown in reverse video. The local module appears in reverse video in the top left hand corner of the screen. Also shown are status flags for the local module. These are summarized in figure(3.1) Flags - bits 0-F 0 - This link contains a valid entry 1 - Primitive module, no subdictionary, never needs compilation and has no text. 2 - Not compiled. This module failed compilation for some reason. 3 - No text. This module was created from the interactive environment, or has had its text striped off. This module cannot be recompiled. 4 - Header created by text, used while compiling from text. 5 - Suitable for inline coding. 6 - Immediate module. 7 - Compiler only module. 8 - Trace mode on. 9 - This module has 8087 alternate code in the parameter field. Figure 3.1 The following is a list of keys and the functions they invoke while in the dictionary editor. Left/right arrow - Selects a module. Up arrow - raises to the parent dictionary, up to the root. If the root module is the local module, this key does nothing. Down arrow - lowers into the dictionary of the selected submodule. Ctrl left/right (IBM PC) or shifted left/right arrows (TI PC) - move the selected submodule left or right. This feature allows the submodules to be re-ordered. Ctrl page up (IBM PC) or shifted up arrow (TI PC) - move the selected submodule up in the dictionary, after the local module in the subdictionary of the submodule previous to it. Ctrl page down (IBM PC) or shifted down arrow (TI PC) - move the selected submodule into the dictionary of the submodule previous to it, moving the submodule farther from the ROOT dictionary. ESC - return to the interactive mode. C - compiles the local module. Compiler errors are possible. If the compiler finds an error, an error message is displayed, and Fifth returns to the interactive mode. Del or D - deletes the selected submodule and all of it's subdictionaries. All submodules after the deleted submodule are marked as not compiled. E- edits the text of the local module. See module Editor. Ins or I - allows a new submodule to be created in front of the selected submodule, or the local module to be (re)created. G - Executes the local module as if typed from the interactive environment. Note that G provides no method of placing values on the stack, thus routines expecting values on the stack will not execute properly in most cases. L - redirects the input stream to the specified file. This allows any text file to become input to Fifth. Text loaded is NOT compiled. However, if the loaded text redefines modules used by the load text routine, unexpected results can occur. Be especially careful about naming modules {UP}, {EDIT}, tilde, and {ABORT}. If these modules are redefined by the text being loaded in, the text load will not work properly. R - renames the local module. Renaming the ROOT module will change the name {SAVE} uses when saving the Fifth system. S - saves the local module and all sub-modules in a format suitable for text editors or listing. This format may be used in conjunction with the L key to upgrade a Fifth program to a higher version number. T - toggles the trace mode on or off. Trace "ON" allows single stepping of Fifth modules. See {TRACE} in the primitives section in the manual or use the online help. X - compiles the local module if nessecary, then strips text from the module and all children of the module. This may be used to free memory when the module will not need recompiling. -- The Text Editor -- The text editor is invoked by the module {EDIT} or from the local dictionary editor, {DIR}. Upon exiting the editor, the compiler is invoked. Thus the Fifth compiler, or any other compiler implemented under Fifth, is incremental. Parts of the program are compiled separately. This gives fast edit-compile-run turn around. The Fifth module editor is simple. Only the most basic text editing functions are provided. Fifth modules are normally quite small, thus a simple editor is normally sufficient. In Fifth the program structure is edited using the dictionary editor, thus providing block operations on functional boundries. The system editor is a very simple full screen text editor. The following is a summary of the key definitions in the editor. ARROW right/left - Move the cursor right/left one character. ARROW up/down - Move the cursor up/down one line. PAGE up/down (IBM PC) or ALT ARROW up/down (TI PC) - page up/down. CONTROL PAGE up/down (IBM PC) or SHIFT ARROW up/down (TI PC) - Drag line up/down. F1 - Insert line. F9 - Split line. CONTROL Y and F7 - Delete line. F8 - Undelete line. F10 - Merge line. INS - Toggle insert/replace mode. The cursor is half height with replace mode, full height with insert mode. CONTROL G and DEL - Delete a character. TAB - Tab. SHIFT TAB - Back tab. CONTROL W - Toggle word wrap. -- Disk I/O -- Disk I/O is through MSDOS 2.00 extended disk I/O calls. Instead of a FCB, a 16 bit file handle uniquely identifies the file. If old-style I/O is required, the catch-all module INT allows a DOS interupt to be used. All I/O calls are subject to error, this is indicated by the error flag and code. The error code is returned by DOS and may be found in a DOS manual.