Translation – Expression Type

The Translator needs to keep track of the expression type as an expression is translated. The expression type is similar to the data type, but not the same. There are two types of expressions, Numeric (the double and integer data types are interchangeable in expressions) and String (including the sub-string data type). The issue was discovered when working on the INPUT PROMPT string expression, but the problem applies to all expressions. Consider these examples that will currently report an error at an inappropriate token:

Z = A$ + B$ + C$
            ^-- expected double

Z$ = A + B * C
           ^-- expected string

These errors will be confusing. The errors in both cases should be pointing to the A$ or A variables and should report “expected numeric expression” and “expected string expression” errors. Using just the word double in the message might imply that an integer expression is unacceptable, which is not the case.

The expression type may need to be set to a generic Any expression type, like would be needed for the PRINT command's expressions or the arguments of non-internal functions. For internal functions, the expression type can be set to the correct value for each argument (using the Numeric expression type if the argument is a double or an integer).

For arrays, the subscripts need to be integers (Numeric), but remember that the Translator does not know if a token with a parentheses is an array or a user function. So the expression type will have to be set to the Any expression type and the Encoder will have to do the checking once it is known whether the token is an array or user function.

Translation – INPUT and Error Reporting

Upon designing the actions required by the add input codes routine, I realized that it may need to point to a different token when an error is detected. Consider this example:

INPUT PROMPT A;B

The error will be “expected string expression” and should be pointing to the A, but the current token being processed is the semicolon token. Therefore, instead of passing the token code of the token being processed, a reference to the token pointer will be passed so that it can be changed to point to the actual token with the error.

For The INPUT command handler (called at end of statement tokens), a Colon code was going to be passed regardless of the actual end of statement token, but now a reference to a token pointer will be passed. This will be whatever end of statement token, which was passed to the INPUT command handler and will be passed on to the add input codes routine.

This led to another problem, consider this example:

INPUT PROMPT A*B+C;D

The + operator will be on top of the done stack after the expression is processed, so the error will be pointing to + since it will have the double data type. The error should be pointing to the A indicating that a string expression was expected. This problem applies to other statements as well, so some sort of expression type detection is needed...

Translator – INPUT Command (Design)

As entries were being written for INPUT with Semicolons, INPUT with Commas, and INPUT Command Handler, there was a lot of copying and pasting of the same text for what the will need to do for translation. In considering how to reduce the amount of words, a realization was made that this is an indication that since the code is the same or very similar that perhaps the common code should be put into a single routine.

The plan was already to have an add input code routine like the add print code routine.  But since there is more common code for the INPUT and INPUT PROMPT commands, the functionality of this add input codes (plural since multiple codes are involved) routine will be expanded.  This routine will need to know the command being translated (INPUT or INPUT PROMPT) along with the current command flags and will need to know which token is being processed (Semicolon, Comma, or an end of statement token).

The first argument then will be the command item from the command stack, which contains the current command code, command flags and the command's token. From the Semicolon and Comma token handlers, this will be the top command item on the command stack.  From the INPUT command  handler, the command item has already been popped from the command stack and is passed as an argument. This is why the add input codes routine can't just use the top of the command stack.

The second argument will just need to be the code for the token being processed.  The Semicolon and Comma token handlers just need to pass their own code.  The INPUT command handler will pass the Colon code, which will be interpreted as the end of statement.  (Noting that the INPUT command handler could be called from any end of statement token like EOL, Colon, ELSE, and ENDIF; so using the Colon code is appropriate.)

Translator – New Token Modes

Translating INPUT statements will require additional token modes. The current token modes are:

Command – Translator is expecting a command token (or start of an assignment)
Assignment – Translator is expecting an item for an assignment statement
EqualAssignment – An equal token was received when the mode was Command or Assignment; another equal token would indicate a multiple assignment statement (commas are not permitted)
CommaAssignment – A comma token was received when the mode was Command or Assignment; another comma token would indicate continuation of a multiple assignment statement (an equal token would indicate the end of the list and the begin of the expression)
Expression – Translator is expecting operands of operators depending on the current state

When a semicolon appears at the end of an INPUT statement, no further tokens should be received except for an end of statement token (EOL, colon, ELSE, and ENDIF). A new mode is required so the Translator can make sure no additional non end of statement tokens are received:

EOS – Translator is expecting an end of statement token only

An INPUT statement contains variable(s) that are to be input. Expressions are not allowed (except for the string expression after the PROMPT keyword, or within subscripts of array variables). The INPUT translation could be implemented to check if the token on top of the done stack has the reference flag set, and if not, report an “expected variable” error. But that could leave to strange errors being reported, consider this example (with the translation of the expression):

INPUT A*B+C  A B * C +

The + will be on top of the done stack after this expression is translated (being the result of the translated expression). The + token will not have the reference flag set since it is an operator. The INPUT is expecting  a reference, so it would report “expected variable” pointing to the + token. This would be very confusing – why would a variable be expected at the +? The correct error should be “expecting comma or end of statement” pointing to the * token. A new mode is required so the Translator can make sure no operators (except for end of expression operators comma, semicolon and EOL) are received:

Reference – Translator is only expecting reference tokens and end of expression operator

Reference tokens include tokens without parentheses and tokens with parentheses. However, these type of tokens could be variables, arrays or functions, but the Translator is not able to determine which. Therefore, the Encoder could still find errors if a user function was placed in an INPUT statement. Lastly, sub-string functions (while valid in assignment statements) are not valid in INPUT statements, therefore the Reference mode needs to check for these and report an error.

Translator – INPUT command

Similar to the translation of the PRINT statement, a lot of the translation work of the INPUT statement will take place in the semicolon and comma token handlers, with the INPUT and INPUT PROMPT command handlers being called at the end of the statement. The codes have been renamed for consistency and clarity, InputGet code will now be InputBegin, and InputPromptStr and InputPrompTmp will now be InputBeginStr and InputBeginTmp (the Tmp versions are not handled by the Translator).

There will be two command flags to keep track of the INPUT statement translation. The first is the InputBegin command flag, which indicates whether an InputBegin has been appended to the output yet. The semicolon and comma token handlers will use this flag to determine if the InputBegin code has been appended (INPUT) and whether a prompt string expression result is expected (INPUT PROMPT).

The semicolon will also use the InputBegin command flag to determine if it is at the end of the statement, which determines whether to set the second command flag, InputKeep. The InputKeep flag will be used by the command handlers at the end of the statement to determine whether the InputKeep sub-code flag should be set in the Input and InputPrompt codes at the end of the translated statement.

Translator – Error Handling (Preliminary Release)

I decided to try using CVS branching for development of new releases. This way, working code (but not ready to release) could be committed to the CVS repository and development can continue. Differences made can then be easily checked. What has been done was good files were copied to a different file name, which is fine for one file, but when a whole set of files are involved, it becomes pain. This accounts for the strange CVS revision numbers in the source file, but should clear up once an official release is made.

Since the known error reporting issues have been resolved, this is a good time to make a preliminary release before the changes to implement the INPUT command begin. There are now many test inputs for testing errors (there should probably be more). The file ibcp_0.1.14-dev-1-src.zip has been uploaded at Sourceforge IBCP Project along with the binary for the program. Now implementation of the INPUT command can begin...

Translator – Error Handling (Corrected)

Between working on the development of the INPUT command this week, some time was spent on working out the remaining error reporting issues. Three major areas needed additional checks to report the desired errors:

1. Internal functions not inside expressions, except for functions that return a sub-string  (which can be used in assignments) – this was an issue because the Translator would allow internal functions in assignments, but only the sub-string functions are allowed.
2. At the end of an expression if the mode is assignment, checks were added to see if the Translator was inside an array or sub-string function – this was an issue because the Translator was in expression mode for the subscripts/arguments, but the errors were related to the assignment, therefore the count stack was checked to determine the current situation.
3. There were several places where checks were needed to determine if the done stack was empty to report the desired error – this included the end of expression in comma assignment mode, comma in assignment mode, semicolon in assignment mode, and LET command with no variable after keyword.

One more check was required at the end of a statement (currently just end-of-line, but will be required for colon when that is implemented). If an EOL was received with the mode set to comma assignment (meaning more items in the list are expected – the mode is set to expression once an equal token is received), then an “expected equal or comma for in assignment” error is reported. A new end statement table entry flag was added for this.

The giant switch statement for the error messages in the test source file was not desirable. These eventually need to be placed elsewhere. Putting them in an array is the logical choice, but how to index the array? One way is to make sure the entries are in the same order as the token status enumeration, but this requires care to make sure it is correct and when it is not, the problem is difficult to debug. So a structure was added that contains the token status value and the string. During initialization, an index conversion array is set up, and during set up is validated to make sure there are none duplicated or missing – just like with the codes enumeration values in the table entries.

Since the error mechanism is almost identical to that used with table errors, instead of cloning this code (a common, but bad, practice of many programmers), an error template structure was implemented to be used for the token status message errors and table errors (the token status message errors only require the duplicate and missing error types). The routine to print the errors in the error list is also defined in the template in the include file. Putting calls to the fprintf() standard library function in this function is a bad idea, so a pointer to generic print function is passed to the error reporting template function. For now the print function passed just outputs to the standard error stream.

Execution – INPUT command (Revised)

Both the INPUT and INPUT PROMPT commands will share routines at run-time. Normally it will not be desirable to call extra functions or check sub-code flags at run-time as this uses execution time (there is overhead calling functions in C/C++). Better if these decisions can be made before hand. But since this is the INPUT command, execution time will not be critical. Therefore, there will be a function for outputting the “? ” (optionally based on a flag argument) and reading the input from the keyboard.

Instead of just pushing the location of the input get code to the evaluation stack (so that it can be re-executed upon an error), a flag for whether there is a prompt expression and a flag whether the prompt is a temporary string will be pushed along side the location. In other words, there will be an input stack item structure containing the location, prompt flag and temporary flag. So when the Input or InputPrompt code is executed, they will have access to whether there was a prompt string (that needs to be popped) and whether it is a temporary string (that needs to be deleted).

But is it necessary to know there is a prompt since Input will know that there is no prompt and InputPrompt will know there is a prompt (though it still needs to know if the prompt is a temporary string). Both Input and InputPrompt will need to check for an EOL in the input buffer, process the assignments on the stack, pop the location entry from the stack, and check if the cursor should be kept on the same line.

Since the temporary flag needs to be carried from the InputPromptStr and InputPromptTmp codes (why not also carry the prompt flag), and both Input and InputPrompt share a lot of the same work, both will share the same run-time routine. This is verses having three routines (one for Input, one for InputPrompt, and one for the common work). The single run-time routine will check for a prompt (which will be popped from the stack) and if it is temporary (which will be deleted).

The InputGet code will push the location with no prompt flag, and then call the get input routine with the question flag argument set. The InputPromptStr will push the location with the prompt flag set and no temporary flag, and then call the get input routine with the question flag argument set if the Question sub-code flag is set. The InputPromptTmp will push the location with the prompt flag and temporary flag set, and then call the get input routine with the question flag argument set if the Question sub-code flag is set. Now back to the implementation of the INPUT statement translation...

Translation – INPUT command (Revised)

Now with the updated syntax to the INPUT statement with the addition of the optional PROMPT keyword, the translation needs to be revised. Since the PROMPT keyword will always follow the INPUT keyword, it will be easiest to handle it as the double keyword command INPUT PROMPT, in other words, it will be a separate command. The translations of these two statements will now be:

InputGet {<variable> InputType}... Input
<expr> InputPromptStr {<variable> InputType}... InputPrompt

This mirrors other commands where the command token is placed at the end of the translation. The InputGet code does not need any sub-codes as it will always output “? ” before getting input. The InputPromptStr will have a Question sub-code to determine if an “? ” should be output after the prompt string.

There will also be an InputPromptTmp code for when the prompt string expression is a temporary string, but this determination won't occur until the Encoder can determine if the string is temporary or not. This also mirrors how temporary strings are handled for other codes. If the Tmp sub-code flag was used, this would be a different scheme and would complicate the Encoder implementation of temporary strings. There is a way to avoid the Input or InputPrompt code needing a Tmp sub-code flag (these are where during run-time that a temporary prompt string is will be deleted).

Both the Input and InputPrompt codes will still have the Keep sub-code for when the cursor should not be advanced to the next line. Here are the revised examples originally shown here with their new translations:

INPUT A InputGet A<ref> InputDbl Input
INPUT PROMPT "Qty: ",A% "Qty: " InputPromptStr A% InputInt InputPrompt
INPUT PROMPT P$+C$,A$; P$ C$ +$ InputPromptTmp A$ InputStr InputPrompt'Keep'
INPUT PROMPT "",A,B% "" InputPromptStr A<ref> InputDbl B%<ref> InputInt InputPrompt

Note there is no equivalent to the last INPUT example command that disabled the prompt – the INPUT PROMPT will have to be used with an empty string as shown above. This should not be an issue since prompt-less INPUT statements will not be common. Also notice that there are three codes that will be outputting a prompt and getting the input. Next, how execution is affected by these changes...

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.