System Function DL
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⎕DL or system function Quad DL is available in monadic form only.
Monadic Function


R is  normally a number greater than zero.  
Z returned by APL's interpreter  is a scalar number, showing exactly how many actual seconds APL was delayed and Z's value is normally somewhat close to input R's value. 
Monadic system function ⎕DL is similar to a sleep or thread sleep routine in other languages.
The following pauses NARS APL program execution for roughly greater than six and a half seconds:
⎕DL 6.651 6.646
R was 6.651 (user specified). Z returned was 6.646. This system function is useful where concurrent/simultaneous processes are running or APL may be dependent on an external routine/program which is running and that external routine needs to finish first, so for example APL can read in the external results. If ⎕DL is used in a user function, say within a loop, every R seconds your APL function can check for that external result, e.g. existence of a file, value within that file, etc.
System Variables (A value may be assigned to these except for ⎕DM)  

⎕ALX  ⎕CT  ⎕DM  ⎕DT  ⎕ELX  ⎕FC  ⎕FEATURE  ⎕FPC  ⎕IC  ⎕IO 
⎕LR  ⎕LX  ⎕PP  ⎕PR  ⎕PW  ⎕RL  ⎕SA  ⎕WSID  
Niladic System Functions (a value cannot be assigned to these)  
⎕A  ⎕AV  ⎕EM  ⎕ET  ⎕LC  ⎕NNAMES  ⎕NNUMS  ⎕SI  ⎕SYSID  ⎕SYSVER 
⎕T  ⎕TC  ⎕TCBEL  ⎕TCBS  ⎕TCESC  ⎕TCFF  ⎕TCHT  ⎕TCLF  ⎕TCNL  ⎕TCNUL 
⎕TS  ⎕WA  
Monadic or dyadic system functions (a value cannot be assigned to these)  
⎕AT  ⎕CR  ⎕DC  ⎕DFT  ⎕DL  ⎕DR  ⎕EA  ⎕EC  ⎕ERROR  ⎕ES 
⎕EX  ⎕FMT  ⎕FX  ⎕MF  ⎕NAPPEND  ⎕NC  ⎕NCREATE  ⎕NERASE  ⎕NINFO  ⎕NL 
⎕NLOCK  ⎕NREAD  ⎕NRENAME  ⎕NREPLACE  ⎕NRESIZE  ⎕NSIZE  ⎕NTIE  ⎕NUNTIE  ⎕STOP  ⎕TF 
⎕TRACE  ⎕UCS  ⎕VR  
Note that quad functions and variables (except for the ⎕A family of functions) are case insensitive 