REPL stands for 'Read Evaluate Print Loop.' The REPL can be used to write and execute code one line at a time and is an alternative to writing code to a file and then compiling or interpreting the entire file before execution.

To start the SMLNJ REPL from a command prompt:

smluser> sml
Standard ML of New Jersey v110.78 [built: Thu Jul 23 11:21:58 2015]
- 3+4;
val it = 7 : int
- (*a comment: press contrl-d to exit *)
smluser>

In the Bash and similar command shells, GNU readline functionality can be added to the SML REPL using the system command rlwrap sml.

smluser> rlwrap sml
Standard ML of New Jersey v110.78 [built: Thu Jul 23 11:21:58 2015]
- 3+4;
val it = 7 : int
- (* pressing the up arrow recalls the previous input *)
- 3+4;
val it = 7 : int
-
smluser>

All SML expressions return a value. The REPL stores the return value of the last evaluated expression. it provides the value of the last evaluated expression within the REPL.

smluser> sml
Standard ML of New Jersey v110.78 [built: Thu Jul 23 11:21:58 2015]
- 3+4;
val it = 7 : int
- it;
val it = 7 : int
- it + 1;
val it = 8 : int
-

[1]+  Stopped                 sml
smluser>

Effectively, comments are not evaluated by the REPL and do not change the value of it.

smluser> sml
Standard ML of New Jersey v110.78 [built: Thu Jul 23 11:21:58 2015]
- 3+4;
val it = 7 : int
- (* a comment *);
- it;
val it = 7 : int

[1]+  Stopped                 sml
smluser>