问题
I'm writing a "script" in Standard ML (SML/NJ) that sets up the interactive environment to my liking. The last thing the script does is print out a message indicating everything went smoothly. Essentially, the last line is this:
print "SML is ready.\n";
When I run the script, all goes well but the SML interpreter displays the return value from the print function.
SML is ready.
val it = () : unit
-
Since I'm merely printing something to the screen, how can I suppress the "val it = () : unit" output so that all I see is the "SML is ready" message followed by the interpreter prompt?
回答1:
To surpress the SML-NJ prompt and response, use the following assignment.
Compiler.Control.Print.out := {say=fn _=>(), flush=fn()=>()};
print "I don't show my type";
I don't show my type
although I don't see why the print function returning the type is bad.
The say function controls what is printed out.
There is a larger example in the following SML/NJ notes http://www.cs.cornell.edu/riccardo/prog-smlnj/notes-011001.pdf
The useSilently function can be used to load a file but without displaying any output associated with the loading
fun useSilently (s) = let
val saved = !Compiler.Control.Print.out
fun done () = Compiler.Control.Print.out := saved
in
Compiler.Control.Print.out := {say = fn _ => (), flush = fn () => ()}
(use (s); done ()) handle _ => done ()
end
This is essentially changing the say function to do nothing and then setting it back at the end.
回答2:
Use this:
val _ = print "I don't show my type";
回答3:
In Moscow ML you can run the REPL without declaration output with
mosml -quietdec file.sml
来源：https://stackoverflow.com/questions/1265171/suppress-val-it-output-in-standard-ml