Invalid index (0) for list: valid range is empty

Question

Hi , 
 I am getting below error the attached code when there I don't pass value for ri!gLLs. Can someone please help in resolving this. 
 
 " Expression evaluation error at function 'remove' [line 67]: Invalid index (0) for list: valid range is empty" 
 a!localVariables(
 /* Appending logged in user with GLLS from LUContacts if logged in user is GLL */
 local!isLoggedInUserGLL: rule!APN_distinct(
 if(
 rule!APN_isEmpty(ri!gLLs),
 {},
 if(
 and(
 not(
 contains(
 touniformstring(
 index(
 a!groupMembers(
 group: cons!RGRACSLBL_GROUP_AGLL,
 direct: true(),
 memberType: "USER"
 ),
 "data",
 {}
 )
 ),
 touniformstring(ri!loggedInUser)
 )
 ),
 or(
 contains(
 touniformstring(
 index(
 a!groupMembers(
 group: cons!RGRACSLBL_GROUP_EU_LL_USERS,
 direct: true(),
 memberType: "USER"
 ),
 "data",
 {}
 )
 ),
 touniformstring(ri!loggedInUser)
 ),
 contains(
 touniformstring(
 index(
 a!groupMembers(
 group: cons!RGRACSLBL_GROUP_WPL,
 direct: true(),
 memberType: "USER"
 ),
 "data",
 {}
 )
 ),
 touniformstring(ri!loggedInUser)
 )
 )
 ),
 append(ri!gLLs, ri!loggedInUser),
 ri!gLLs
 )
 )
 ),
 /* Formatting GLLs text */
 local!glls: if(
 length(local!isLoggedInUserGLL) = 1,
 rule!APN_displayUser(local!isLoggedInUserGLL),
 joinarray(
 a!forEach(
 items: remove(
 rule!APN_distinct(local!isLoggedInUserGLL),
 length(
 rule!APN_distinct(local!isLoggedInUserGLL)
 )
 ),
 expression: split(rule!APN_displayUser(fv!item), ";")
 ),
 ", "
 ) & " and " & rule!APN_displayUser(
 difference(
 rule!APN_distinct(local!isLoggedInUserGLL),
 remove(
 rule!APN_distinct(local!isLoggedInUserGLL),
 length(
 rule!APN_distinct(local!isLoggedInUserGLL)
 )
 )
 )
 )
 ),
 if(
 contains(
 /* CCN */
 {
 cons!RGRACSLBL_NOTIFICATION_TYPES[1],
 cons!RGRACSLBL_NOTIFICATION_TYPES[16]
 },
 ri!notificationType
 ),
 joinarray(
 insert(
 split(
 "Please contact[GLL]directly for any questions regarding this CCN.",
 "[GLL]"
 ),
 local!glls,
 2
 ),
 " "
 ),
 if(
 contains(
 /* Pre-sMA, Revised Pre-sMA */
 {
 cons!RGRACSLBL_NOTIFICATION_TYPES[7],
 cons!RGRACSLBL_NOTIFICATION_TYPES[25]
 },
 ri!notificationType
 ),
 substitute(
 cons!RGRACSLBL_PRE_SMA_AGLL_RICH_TEXT_CONSTANTS[7],
 "[GLL]",
 local!gLLs
 ) & " " & cons!RGRACSLBL_PRE_SMA_AGLL_RICH_TEXT_CONSTANTS[8],
 if(
 contains(
 /* Core SLU */
 {
 cons!RGRACSLBL_NOTIFICATION_TYPES[18],
 cons!RGRACSLBL_NOTIFICATION_TYPES[6]
 },
 ri!notificationType
 ),
 "Please contact " & local!glls & " directly for any questions regarding this update.",
 if(
 ri!notificationType = cons!RGRACSLBL_HQ_INITIATED_NON_SAFETY_LABEL_UPDATE_NOTIFICATION_TYPE,
 /* Non-Safety */
 joinarray(
 insert(
 split(
 cons!RGRACSLBL_TEXT_HQ_NONSAFETY_FREETEXT,
 "[GLL]"
 ),
 local!glls,
 2
 ),
 " "
 ),
 if(
 contains(
 /* Advisory */
 {
 cons!RGRACSLBL_NOTIFICATION_TYPES[9],
 cons!RGRACSLBL_NOTIFICATION_TYPES[17]
 },
 ri!notificationType
 ),
 joinarray(
 insert(
 split(
 "Please contact[GLL]directly for any questions regarding this Advisory.",
 "[GLL]"
 ),
 local!glls,
 2
 ),
 " "
 ),
 {}
 )
 )
 )
 )
 )
)

Mike Schmitt · Answer

Instead of this impossible mesh of spaghetti including questionable uses of remove(), split(), etc, which I suspect is only being used to format the list like "Name 1, Name 2 and Name 3", I suggest you actually utilize the functionality found in a!forEach to do that work for you. The usage pattern I suggest is as follows. Note that you'd merely need to add back in your "displayUser" function here around "fv!item". 
 a!localVariables(
 
 local!loggedInUsers: {"user1", "user2", "user3"},
 
 concat(
 a!forEach(
 local!loggedInUsers,
 if(
 fv!isFirst,
 "",
 if(
 fv!isLast,
 " and ",
 ", "
 )
 ) & fv!item
 )
 )
) 
 Note that this will work both for multi-member lists as well as one-member lists, inherently, without needing to resort to such messy tactics as found in the original code.

Mike Schmitt · Answer

Additionally I'd suggest that if a list of distinct users is necessary to use here, then instead of calling APN_distinct() over and over again, simply create the list of distinct users in a local variable once and use that. 
 Instead: a!localVariables(
 
 local!loggedInUsers: {"user1", "user2", "user1", "user3"},
 
 /* use Distinct function ONCE */
 local!distinctUsers: rule!APN_distinct(local!loggedInUsers),
 
 concat(
 a!forEach(
 local!distinctUsers,
 if(
 fv!isFirst,
 "",
 if(
 fv!isLast,
 " and ",
 ", "
 )
 ) & fv!item
 )
 )
)

Invalid index (0) for list: valid range is empty

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