Language – INPUT command (Change)

As the translation of the INPUT command was being designed, a serious flaw was uncovered with the current syntax. This can be shown with an example. Say there is a user function named Prompt$ that takes a single argument, in other words FUNCTION Prompt$(Index). The intention is to use this function as the prompt string expression in an INPUT statement, which looks like this (using the comma separator to suppress the addition of the default question mark prompt):

INPUT Prompt$(I),Array(I)

To the Translator, the Prompt$ is a token that has a parentheses, which could be either an array or a user function. Requiring parentheses to be inserted around Prompt$(I) to force it to be detected as a prompt expression instead of a variable is not acceptable. The translation of the above is different depending on whether it is an array or a user function:

Array:  INPUT'Question' I Prompt$(<ref> InputStr I Array(<ref> InputDbl Input
Function:  I Prompt$( INPUT'Prompt+Question' I Array(<ref> Input Dbl Input'Tmp'

These two translations are radically different, so one can't assumed and then changed later by the Encoder when it has determined whether it is an array or a function. The Translator needs to know whether it is a prompt or not. If it knows that it is a prompt, then it does not need to know whether it is an array or a function (the translations are the same). For a semicolon, this is not an issue because the item before semicolon is a prompt (unless it is at the end of the line).

To resolve this issue, there are two alternatives. Either the comma separator be eliminated (where a semicolon only indicates a prompt and suppresses the “? ” leaving no way to add the “? ” except to actually add it to the prompt – not desirable), or use the ANSI BASIC syntax of the PROMPT keyword to indicate a prompt string expression follows. The latter will be used and the functionality of the comma (no “? ”) and semicolon (add “? ”) will remain.

Using the PROMPT keyword is the more logically because it clearly shows that there is a prompt, and this keeps with the spirit of a beginner's language (which is probably why it is part of ANSI BASIC). Therefore the syntax of the INPUT statement will now be:

INPUT [PROMPT <string-expression> {,|;}] <variable>[,<variable> ...][;]

Next, how this will affect the resulting translation and execution of the INPUT statement...

Execution – INPUT command (More Details)

The characters in the input buffer are parsed into tokens separated by commas. If there is a double quote at the beginning of a token, then a string constant is scanned for a matching closing parentheses. The surrounding double quotes will not be included in the string value. Two double quotes within the string constant are treated as a single double quote. If the token begins with a numeric constant character, then an attempt is made to see if the entire token is a valid double or integer constant (otherwise it is considered a string constant). After each value token is parsed, the status flag is set to Comma or EOL depending on what terminated the value.

The InputDbl code will check for a double or integer value, otherwise an error occurs. An integer constant is converted to a double. The InputInt code will check for a double or integer value, otherwise an error occurs. A double constant is converted to an integer if possible, otherwise an error occurs. An InputStr code will accept any value type. For double and integer values, the characters entered are used. Double quotes are needed around a string value that contains a comma.

When an error occurs, everything pushed to the evaluation stack needs to be popped up to the prompt (if there is one) and execution needs to repeat the INPUT statement. The reason the location of the InputGet code is pushed to the stack is so that execution can resume there upon an error. Any string prompt on the stack is not popped so that it can be reused.

The code the error occurs at (an InputType or Input) and the input counter value will determine how many entries need to be popped from the stack. For an InputType code, an error would occur after the variable reference is pushed to the stack but before the value and assignment function pointer. For InputType, the number of entries to pop is count*3+1. For Input, the number of entries is count*3. Once these entries are popped, the location of the InputGet is popped. The string “Redo from start” is output and execution continues at the location of the InputGet.

Notice that at no time during the execution of the INPUT statement, including error processing, is it required to know what type of value is at any stack entry – it already knows. Next, onto the translation of the INPUT statement...

Execution – INPUT command

To prove that the selected translation is appropriate, consider what occurs during execution, here are some example INPUT statements along with their translations:

INPUT A  InputGet'Question' A<ref> InputDbl Input
INPUT "How many: ",A%  "How many: " InputGet'Prompt' A%<ref> InputInt Input
INPUT P$+C$,A$;  P$ C$ +$ Input'Prompt' A$<ref> InputStr Input'Tmp+Keep'
INPUT,A,B%  InputGet A<ref> InputDbl B%<ref> InputInt Input

If there is a prompt string expression, its result will be on top of the evaluation stack. At the InputGet code, if the Prompt sub-code flag is set, then the string on top of the stack is output. If the Question sub-code flag is set, then a “? ” is output. The location of the InputGet code is pushed to the stack (to be used if an error occurs so that the InputGet can be returned to). The characters are input into a buffer until an Enter key terminates the input. There will be a status flag variable to keep parsing status of the input (initialized to None) and a value counter variable to count the number of input values (initialized to zero).

A variable reference is pushed to the evaluation stack. At an InputType code, the status flag is checked for a None or a Comma status, otherwise an error has occurred. The input buffer is scanned for a value token. If the value is the correct type, or can be converted to the correct type, then it is pushed to the stack, otherwise an error occurs. Finally, a pointer to an assign code run-time function for the data type is pushed to the stack.

At the Input code, the status flag is checked to make sure that it is set to EOL (a Comma status indicates there is more input, which is an error). An assign function pointer is popped from the evaluation stack and then called, which assigns the value to the variable that is the stack (both are popped by the assignment). This repeats for each value until the input value counter is decremented to zero. The InputGet code location is popped from the stack and finally the prompt string is popped from the stack. If the TmpStr sub-code flag is set, then the temporary prompt string is deleted. If the Keep sub-code flag is not set, the Print function is called to advance to the next line.  Next, some more execution details...

Translation – INPUT command

Without going the entire process in how the translation was arrived at, here is the general form of the translated INPUT statement (optional items in square brackets, repeating items in braces):

[<expr>] InputGet {<variable> InputType}... Input

The InputGet code will output an optional prompt string (evaluated by the preceding string expression and pushed to the evaluation stack) and the “? ” if selected, and then will get the input. The InputType codes will parse the input for a appropriate data type. Any errors cause a loop back to the InputGet code (where the prompt is issued again). When there are no errors, the Input code will assign the values to the variables, pop the prompt string from the evaluation stack, delete the string if it is a temporary, and advancing the cursor to a next line if not suppressed by a trailing semicolon.

The InputGet code will use sub-code flags to indicate whether there is a prompt string to output and whether to output the “? ” string. The Input code will use sub-code flags to indicate if there is a temporary prompt string that needs deleted and whether the cursor should be kept on the same line.

Normally sub-code flags are only used by the Recreator when generating the original source, and ignored during run-time (so as not to waste execution time checking them). But, considering that this is the INPUT command, a command that is about to stop and wait for an extremely slow user, using time to check sub-code flags is not going to have any significant impact. Next, a more detailed look at how the INPUT command will be executed...

Internal Code – INPUT command (Revised)

Upon considering the resulting translation of the INPUT statement, I realized there is an efficiency that can be made. The prompt string expression only needs to be evaluated one time. The resulting string can be left on the evaluation stack until the end of the INPUT statement. This way, if the prompt is needed again due to an input error, the string expression result is available and does not need to be evaluated again.

This means that the PrintStr or PrintTmp code can not be called because both will pop the string from the evaluation stack and PrintTmp will delete a temporary string. After the values are assigned, then the prompt string can be popped from the evaluation stack and deleted if it is a temporary.

There is one other aspect of the INPUT statement that needs to be considered. There is a built-in looping required to deal with errors. Not having to re-evaluate the prompt string expression means that the loop back jump does not have know where string expression codes begin. Next, what the INPUT translation will look like...

Internal Code – INPUT command

Before the INPUT command can be implemented in the Translator, the translation of the INPUT statement translation must be determined; and before this, how the INPUT command will be executed at run-time. The steps of the INPUT command at run-time are:

1. Print the prompt string is present
2. Print the “? ”  prompt (if selected)
3. Get the input
4. Parse a value from the input for the data type of the variable
5. Save the value and repeat for each variable
6. If there is an error with a value or to many or too little values
7. Then print “Redo from start” and go back to the first step
8. Assign each variable to its corresponding value
9. Move the cursor to the next line (unless there was a semicolon at the end)

The run-time code for the PrintStr code can be called to output the prompt string (step 1). If the prompt is a temporary string, then PrintTmp needs to be called (identical to PrintStr except deletes the temporary string when done). But like all other code that take string operands, the exact code can't be determined by the Translator.

Like with the PRINT statement, there will be separate codes for parsing the input for a particular data type - InputDbl, InputInt, and InputStr (step 3). The values can't be assigned until all the values have been parsed and are valid, so the values need to be saved. Pushing them onto the evaluation stack right after the variable's reference is the logically thing to do.

Once all the values are input, the values can be assigned to the variables (step 8). The run-time code for the Assign data type can be used since the values will be on the stack (variable reference first and then data value). If there no semicolon at the, the cursor needs to be advanced to the next line. The run-time code for Print can be used (step 9).

Language – INPUT command (More Details)

Allowing a string expression for the prompt will give flexibility. However, if using a single string variable for the prompt's string expression can lead to possible confusion. Consider these INPUT statements where the variable P$ is meant to contain the prompt string:

INPUT P$;A,B
INPUT P$,A,B
INPUT A$;

For the first statement, it is easy to determine that the P$ is a prompt since a semicolon is separating the prompt from the input variables. However, in the second statement, is the P$ the prompt or is the first of the three variables to input? The later will be assumed because otherwise there would be no way to input a string as the first of two or more variables.

The Translator will determine if the first parameter is a string expression or a string variable by checking if the reference flag of the token is set. The reference flag will not be set for an actual string expression. To override this behavior, the statement can be written as “INPUT (P$),A,B” where the parentheses will clear reference flag of the P$ token forcing it to be interpreted as a string expression and therefore as a prompt.

In the third statement, since no variables follow the semicolon, this statement will be interpreted as inputting a single string variable, keep the cursor on the same line after the input is entered.

I have been doing some reconfiguring/upgrading of my computer (adding a larger and faster hard drive), which didn't go as smooth as it should have. At the same time a board was removed and the system wasn't stable until it's drivers were removed (duh). And there was an error on the system partition that proved to be a great pain to correct until the correct procedure was figured out. Finally the system partition corrected, defragmented and copied to the new hard drive. Work can now continue on this project.

Language – INPUT command

After looking at QBasic (GW-Basic, QuickBasic and FreeBASIC have the same features on the INPUT command) and TrueBASIC (similar to ANSI Basic) for inspiration, a set of desired features for the INPUT command was selected. There's no reason to get too elaborate with the INPUT command as there is a better solution for inputting values (more on that later). The syntax chosen for the INPUT command implementation is:

INPUT [[<string-expression>] {,|;}] <variable>[,<variable> ...][;]

QBasic (and GW-Basic, etc.) supports an optional prompt string, but it must be a string constant. TrueBASIC (ANSI BASIC) allows a string expression but it is preceded by the PROMPT keyword (followed by a colon and the "? " is suppressed). ANSI BASIC has some other options in additional to PROMPT (perhaps TrueBASIC does now too, but not in the 1985 version).

A string expression will be used for the prompt so that anything can be used for the prompt, not just a string constant (which is too restrictive). Similar to QBasic, a comma separator after the prompt outputs just the value of the string expression, and a semicolon will add "? " after the prompt. If there is no string expression or separator, then the standard "? " is output. The prompt is optional, and if there is just a separator, then there will just the "? "  (semicolon) or no prompt (comma). Using just the semicolon is pointless because it is the same as not having the semicolon. Using just the comma will have no prompt (which gives errors with both QBasic and GW-Basic).

The trailing optional semicolon will keep the cursor on the same line after the input is entered. QBasic (and the others) accomplish this with the optional semicolon directly after the INPUT keyword. This is weird and it makes more sense to put the optional semicolon at the end just like with the PRINT statement.

Note: Debugging will resume on the error reporting issues, but I have little time available during this week and wanted to get started with the background for the INPUT command.

Translator – PRINT Command (Release)

The original plan was put all the command test inputs into one set, but the set grew quite large when the PRINT tests were added and especially when all the error test inputs were also added. Therefore, new separate sets were created for the PRINT tests and the error tests. The lone PRINT statement at the end of the LET command tests remains (though it no longer causes a “BUG: not yet implemented” error).

One last thing, it was previously mentioned that the standard C libraries being used with GCC include a command history of the inputs entered in the interactive test input mode, accessed by the up and done arrows. I accidentally discovered a few more features using the function keys (found when F7 was entered in the MSYS window instead of the VIDE2 window where it is the build program shortcut key). These features are:

F1 – Copy one character from previous input at current position
F2 – Pops up a window asking for a character, then copies characters from current position from previous input up to that character
F3 – Copies all characters from previous input at current position
F4 – Pops up a window asking for a character to delete to, then deletes up to the character
F5 – Does the same thing as up-arrow
F6 – Puts Control-Z into buffer (not sure what this is for, possibly Control-Z is an end of file marker from the old DOS days)
F7 – Pop up a window showing are previously entered inputs (each numbered), which can then be selected
F8 – Display previous input (same as up-arrow except rotates around, but only when cursor is a beginning of line)
F9 – Pop up window asking for a Command Number

The Page Up and Page Down go to the first and last inputs saved. I also noticed it seems to remember the inputs even between runs, I'm not sure where it stores this between terminating the program and starting it back up.

Anyway, the Translator now has support for the PRINT command and PRINT functions and ibcp_0.1.13-src.zip has been uploaded at Sourceforge IBCP Project along with the binary for the program. A lot of changes were required to get the PRINT command implemented, the implementation of next command (INPUT) should be easier. Now to get the rest of the error handling working correctly...

Translator – PRINT Working, But...

This release seems to be living up to its unlucky number. I'm going to just put it out shortly and move on. The PRINT command and its error tests are working properly, so there's no reason to do an interim developmental release.  However, not all the error messages are correct.  But first, a few changes need to be described.

A while back, an AssignList flag was added to table entries to identify that the code is an assign list code. This flag was used when processing an operator to identify it as an assign list operator. A temporary flag is set if the operator is an assign list operator before the find code function was called (which only handles the last value in the assignment list and the expression). Upon returning, if this temporary flag is set, then the rest of the assignment list operands are processed.

It turns out this table entry flag is not necessary. Since the temporary flag is set before the find code function (which may change the operator code depending on the data types), the temporary flag can simply be set if the operator code is AssignList.

One other change was made related to list assignments. Previously the AssignList operator was not pushed to the hold stack until the equal token was received, the comma tokens were just deleted. This was changed to push an AssignList Operator token to the hold stack when the first comma token is received (the comma token was changed). This occurs when the mode is changed from Assignment to CommaAssignment (on the first comma token, subsequent comma tokens are still deleted). When the equal token is received, the mode is still changed from CommaAssignment to Expression (but the equal token is now just deleted). This change was necessary to aid in identifying which specific error message is needed when an error occurs (which is still not working correctly).

Translator – PRINT Command (Still Testing)

Wow, as the remaining errors were corrected, I thought of more errors to try, and of course they did not work either and needed to be corrected. Right now there are three more errors that are not being reported correctly (and none of these were known before this evening). All of the previously known error reporting issues had been corrected plus a few more that were discovered. The various error test inputs were put into there own group, which has grown to 79 different tests (though there maybe a few duplicates). Hopefully one more day...

Translator – PRINT Command (Testing)

Sorry for the lack of posts but it has been quite an effort trying to get the desired error messages for the different errors. The simple approach would be to simply report “Syntax Error” like GW-Basic does for errors, but that defeats the idea of a Beginner's programming language – it should be helpful. After seeing how well QBasic identifies errors (though not perfect), the decision was made to do an even better job than QBasic.

There has been a little backtracking on the scheme that all error codes (and messages) should be unique. There was starting to be a lot of similar messages except for subtle differences like a “; not semicolon” or “; not comma” at the end. The decision was made that this was unnecessary, since the location in the code could be identified by the token being pointed to (comma, semicolon, or EOL), so there's really no point to having multiple unique errors for these.

It would require too much space to try an explain all the details of the changes, but here is a summary of the major changes:

1. Print-only functions are checked if invalid immediately upon receipt instead of waiting until the function is processed at the closing parentheses (without this, a statement like A=TAB(B$) would report that the argument is invalid possible implying that the TAB is okay in the assignment).
2. The counter stack was modified to hold both the operand counter it previously held and a new number of expected arguments value that is set for internal functions. This value is used to determine if another argument is expected or no more arguments are expected for a given internal function.
3. The comma and semicolon token handlers were reorganized for better error reporting.
4. The expression end check function was modified to check if a comma or closing parentheses is expected for internal functions (complicated by functions having different number of arguments).
5. Many checks of the current mode were required in the different locations in the Translator code so the correct error could be reported.

There is still a few errors that are not being reported as desired. A little more work is required to clean these up and a release can finally be made...

Translator – PRINT Command (Testing)

A statement like “Z=A+” was reporting the “BUG: expected operand on done stack” error because the add operator (emptied at the EOL) consumed two operands from the done stack, the A and then the Z. The EOL empties the assignment operator next, which is expecting two operands, but there  was only one, the result of the add operator, thus the bug error.

To fix this problem, a new FirstOperand state was needed. This state is set at the beginning. After the first operand, the Operand state is used (except after an assignment operator, comma in a PRINT statement, and semicolon in a PRINT statement). When an operator is received in operand mode that has the end expression flag set (comma, semicolon and EOL), only if the state is not FirstOperand (meaning operands have been received yet) does an error occur. The error message was changed from “operand expected” to “expected expression” since more than just an operand is expected (unary operator, parenthetical expression, etc.).

Upon more comparing with Qbasic's error checking behavior, it became evident that the current error messages could be more helpful in explaining what is expected. Therefore many of the error messages were updated and some new messages were added. In keeping with the current scheme, all error codes (and messages) are unique – in other words, occur in only on location. This makes it much easier to identify what part of the code generated the error.

Many more test inputs were added with all kinds of errors to see if the error messages produced was sufficient and pointing to the proper token. The same statements were tried with QBasic. In some cases, I feel the QBasic message is insufficient. There was even one error statement “A B = 1” that QBasic did not catch as an error until the program was run (oops). There are still more adjustments to the error messages required (including two bug errors that are still occurring and two that are not pointing at the correct token). Hopefully this will be cleaned up shortly and the next release, with the PRINT command, can be made...

Translator – PRINT Command (Still Debugging)

And speaking of QBasic, out of curiosity of how it handles invalid commas in function arguments for comparison to IBCP, QBasic detects the wrong number of arguments at the first comma that shouldn't be there (the “expected )” error occurs pointing to the comma) or the closing parentheses when there should be another argument (the “expected ,” error occurs pointer to the closing parentheses. IBCP doesn't catch the error until the closing parentheses (and points to the function name with the “wrong number of arguments” error). The QBasic behavior is better.

This behavior was not difficult to replicate. The count stack (that keeps track of operands, i.e. commas) was modified to hold a count stack item, which contains the current number of operands (commas) counter and a new expected number of arguments that is set if the token is an internal function. The comma token handler then checks if too many arguments have been entered if the expected number of arguments is set.

To make this work with internal functions that have multiple number of arguments (MID$, INSTR and ASC), the entries in the table needed to be rearranged where the code with the more number of arguments moved first, so that the number of expected operands was set high enough not the have an error reported before getting to the closing parentheses.

One more bug needs to be fixed, a statement like “Z=A+” reports the “BUG: expected operand on done stack” error. Bug errors are programming errors that should not occur. The statement “Z=(A+” does correctly report the “operand or closing parentheses expected” error. This appears to be the final bug...

QBasic (Another Incremental Compiler)

I discovered that QBasic 1.1 is available as a free download. Version 1.0 came with DOS 5.0 as a replacement for GW-Basic and with Windows 95, NT 3.x and NT 4.0. Version 1.1 came with DOS 6.x, Windows 95, 98, and Me. QBasic was based on QuickBasic 4.5. I was fascinated with the features of QuickBasic 4.5 when it came out, the incremental compiler aspect was intriguing, but did not think they took the user interface concepts far enough (one of the intentions of IBCP).

After getting familiar with QuickBasic 4.5, I never used it much afterward since it's compiled programs were much slower than when compiled with QuickBasic 3.0 (not an incremental compiler) for the project I was involved with at the time and continued to use GW-Basic for development and the QuickBasic 3.0 compiler for production.

QBasic does not contain any of compiler or linker elements of QuickBasic 4.5, however, the incremental compiler remains. The lines are compiled as they are entered and errors are reported immediately. QBasic is good inspiration for IBCP and will be used for comparison, both for features and run-time (though possibly unfair since QBasic is not 32-bit). QBasic will be mentioned frequently from here on (along with GW-Basic, QuickBasic 3.0 and FreeBASIC), though there is no intention of making IBCP a clone of QBasic (see FreeBASIC for that, which is just a compiler).

Translator – PRINT Command (More Debugging)

A dummy semicolon token was being appended after a print-only function, which is not necessary. A new semicolon sub-code flag was added for this case along with a new print function command flag, which is set when a print-only function is appended to the output. If the semicolon token handler sees this command flag set, it will set the semicolon sub-code flag of the print-only function token (the last token appended to the output). This command flag is cleared by the semicolon and comma token handlers to prevent further semicolon from seeing it set.

This change was also necessary for print-only functions at the end of a statement, because there was no translated difference between a print-only function at the end or one followed by a semicolon.  To reproduce the ending semicolon, the semicolon sub-code flag is set when there is a semicolon after the print-only function at the end of the statement (the semicolon is not necessary since the print-only functions at the end keep the cursor on the same line).


The check for the end expression flag (set for comma, semicolon and EOL tokens) was put at the beginning of the state is operand or operator check to skip this section when the flag is set. The check was necessary because otherwise an “operand expected” was occurring when a comma or semicolon followed another comma or semicolon, or one of these was at the end of the statement. In this case, the Translator was expecting an operand when the comma, semicolon or EOL was received.

After several failed attempts to get this right, the check was removed to see what error came up for the different test input errors that have been added to test all the possible cases. In all cases, the “operand expected” occurred. This pointed to the location where the end expression flag needed to be.

The location for the check was in the operand section between the “is not an operator” and the “is a unary operator” sections (remember that unary operators are accepted when the Translator is expecting an operand). Making the comma, semicolon and EOL codes unary operators to get is past this section did not work (they were processed incorrectly later in the code), so a separate check was added.

For the end expression token check, a new “operand or closing parentheses expected” error was added (neither “operand expected” nor “missing closing parentheses” made sense for all cases where this error can occur). The error only occurs if the counter stack is not empty (inside a parenthetical expression or inside an array or function), otherwise the error will be caught by the respective token handlers. Debugging continues...