vfp-conventions

name	vfp-conventions
description	Visual FoxPro coding conventions and patterns. Use when writing or reviewing VFP code, understanding naming conventions, variable prefixes, or VFP-specific patterns.
allowed-tools	Read, Grep, Glob

VFP Coding Conventions

Naming Conventions

Database Field Prefixes (CRITICAL!)

Prefix	SQL Type	VFP Type	Example
id*	INT IDENTITY/INT	Integer	idperson, idclient
d*	DATE	Date	dbirthdate, ddeadline
t*	DATETIME	DateTime	tlogon, tcreated
n*	NUMERIC	Numeric	npercent, nprice
c*	VARCHAR/CHAR	Character	cname, ccode
l*	BIT	Logical	lactive, lbillable

Variable Prefixes

Prefix	Scope	Example
gl_*	Global (PUBLIC)	gl_nSQLHandle, gl_cUser
gll_*	Global saved to .mem file	gll_cExportPath
m.*	Memory variables (Scatter)	m.cname, m.ddate
lc*	Local Character	lcSQL, lcWhere
ln*	Local Numeric	lnTotal, lnCount
ld*	Local Date	ldStart, ldEnd
ll*	Local Logical	llSuccess, llFound
la*	Local Array	laResults, laFiles
lo*	Local Object	loForm, loException

Function Prefixes

Prefix	Purpose	Example
sql_*	SQL Server helpers	sql_exec(), sql_update()
log_*	Logging functions	log_error(), log_info()
get_*	Data retrieval	get_client(), get_settings()
set_*	Data modification	set_status(), set_config()

Code Patterns

1. VFP Does NOT Have ELSEIF!

*-- CORRECT - Use DO CASE
DO CASE
CASE nResult > 0
    RETURN .T.
CASE nResult = 0
    RETURN .T.
OTHERWISE
    RETURN .F.
ENDCASE

*-- WRONG - VFP has no ELSEIF!
IF nResult > 0
    RETURN .T.
ELSEIF nResult = 0   && COMPILE ERROR!
    RETURN .T.
ENDIF

2. SCATTER for Read, REPLACE for Write (NOT GATHER!)

*-- CORRECT - Scatter to read, REPLACE to write specific fields
SELECT clients
Scatter Memvar
m.cname = "New Name"
REPLACE cname WITH m.cname
TABLEUPDATE(.T.)

*-- WRONG - Gather sends ALL fields (risky!)
Scatter Memvar
m.cname = "New Name"
Gather Memvar  && May overwrite fields you didn't intend to change!

3. File Operations - Use STRTOFILE

*-- CORRECT - STRTOFILE for append
=STRTOFILE(lcText + CHR(13)+CHR(10), lcFile, .T.)

*-- WRONG - Fopen/Fputs/Fclose is fragile
lnHandle = Fopen(lcFile)
=Fputs(lnHandle, lcText)
=Fclose(lnHandle)

4. NULL Handling

*-- CORRECT - Check type before comparison
IF VARTYPE(ddeadline) = 'D' AND ddeadline < DATE()
    lcStatus = "Overdue"
ENDIF

*-- WRONG - Will crash if field is NULL
IF ddeadline < DATE()  && Error if NULL!
    lcStatus = "Overdue"
ENDIF

5. Date Comparisons for "Overdue"

*-- CORRECT - < for strictly overdue (excludes today)
IIF(ddeadline < DATE(), "OVERDUE", "OK")

*-- WRONG - <= includes today (not overdue yet!)
IIF(ddeadline <= DATE(), "OVERDUE", "OK")

6. Error Handling with TRY/CATCH

TRY
    *-- Risky operation
    USE sometable EXCLUSIVE
    *-- Process data
CATCH TO loEx
    *-- Handle error
    log_error(loEx.Message + " in " + loEx.Procedure)
    MESSAGEBOX("Error: " + loEx.Message, 16, "Error")
FINALLY
    *-- Cleanup (always runs)
    USE IN SELECT("sometable")
ENDTRY

7. Safe Object Property Access

*-- CORRECT - Check object exists first
IF VARTYPE(loObject) = 'O' AND !ISNULL(loObject)
    lcValue = loObject.Property
ENDIF

*-- WRONG - Will crash if object is NULL or doesn't exist
lcValue = loObject.Property

Common Errors

Error	Cause	Solution
"Unrecognized command verb"	Used ELSEIF	Replace with DO CASE
"Operator/operand mismatch"	NULL comparison	Use VARTYPE() first
"Variable not found"	Missing m. prefix	Use m.varname with Scatter
"Property is read-only"	Modifying .Name	Some properties can't change at runtime

Best Practices

1. Always use m. prefix with SCATTER MEMVAR variables
2. Prefer local variables (lc*, ln*) over public (gl_*)
3. Close cursors explicitly - don't rely on scope cleanup
4. Use TABLEUPDATE() after modifying buffered cursors
5. Handle NULL values explicitly in comparisons
6. Use TRY/CATCH for operations that might fail
7. Log errors to help with debugging

Quick Reference

*-- String functions
ALLTRIM(string)      && Remove leading/trailing spaces
UPPER(string)        && Convert to uppercase
SUBSTR(string,n,m)   && Extract substring
AT(search, string)   && Find position

*-- Date functions
DATE()               && Current date
DATETIME()           && Current datetime
DTOC(date)           && Date to character
CTOD(string)         && Character to date
DTOS(date)           && Date to string (YYYYMMDD)

*-- Type checking
VARTYPE(var)         && Returns type letter (C,N,D,L,O,U,X)
TYPE("var")          && Returns type (evaluates expression)
ISNULL(var)          && Check if NULL
EMPTY(var)           && Check if empty

*-- Array functions
ALINES(array, string, .T., CHR(13)+CHR(10))  && Split string to array
ALEN(array, 1)       && Number of rows
ASCAN(array, value)  && Find value in array

Resources

• VFPX Community
• VFP Language Reference
• FoxBin2Prg - Version control for VFP