Interactive Programming using the REPL

Syntax#

Unlike source code files, the semicolon ’;’ is mandatory to terminate each expression in the REPL.

Starting the SMLNJ REPL

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>

Using ‘it’

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>