How to write multiline strings in SML? - string

Question

How can I write a string that spans multiple lines?
I tried to use the line continuation character \ at the end like many other languages, but it does not seem to work. The following:
val s = "line1\
line2";
generates an error:
/tmp/...:1: error: unexpected character l in \ ... \
/tmp/...:1: error: no matching quote found on this line
Not using any line continuation as follows generates a similar error:
val s = "line1
line2";
What's the correct way to write a multiline string?

Answer 1

You have to put the backslash at the beginning of the next lines as well :
print "this is\
\ a string on\
\ 3 lines\n";

Answer 2

If you're using SML/NJ and you don't care much about portability, there's a lesser known feature that could give you not only multiline strings, but also variable interpolation. SML/NJ supports an extensible quotation mechanism, which is described in more detail here: https://www.smlnj.org/doc/quote.html
Now, to solve your problem, we could define the following quotation parser. I'm calling it s to draw a parallel with Scala's s"..." syntax, which will become clearer in a few moments.
val s : string SMLofNJ.frag list -> string =
let
fun fold (SMLofNJ.QUOTE s, acc) = acc ^ s
| fold (SMLofNJ.ANTIQUOTE s, acc) = acc ^ s
in
List.foldl fold ""
end
Notice that it makes use of the SMLofNJ structure to access compiler-specific datatypes and values (frag, QUOTE and ANTIQUOTE).
We can now use the above like this (make sure you're using the command line option -Cparser.quotations=true):
$ sml -Cparser.quotations=true
- val a = "var";
val a = "var" : string
-
- val b = `this is a raw quotation using ^a`;
val b = [QUOTE "this is a raw quotation using ",ANTIQUOTE "var",QUOTE ""] :
string SMLofNJ.frag list
-
- val c = s`this is an interpolated string: ^a`;
val c = "this is an interpolated string: var" : string
-
- print c;
this is an interpolated string: varval it = () : unit
-
- val d = s`this
… is
… a
… multiline string: ^a`;
val d = "this\nis\na\nmultiline string: var" : string
-
- print d;
this
is
a
multiline string: varval it = () : unit
As you can see, using quotations in conjunction with a very simple custom parser (the s function), we can get multiline interpolated strings in SML/NJ.
In my personal library, I've defined it in a bit more structured way:
structure Strings =
struct
structure Interpolated =
struct
local
open SMLofNJ
in
(**
* Support for multiline, interpolated strings.
*
* NB: requires SML/NJ quotations: sml -Cparser.quotations=true.
*
* ```sml
* - open Strings.Interpolated;
* - val v = "val";
* val v = "val" : string
* - val j = s`{
* = "key": "^v"
* = }`;
* val j = "{\n \"key\": \"val\"\n}" : string
* ```
*)
val s : string frag list -> string =
let
fun fold (QUOTE s, acc) = acc ^ s
| fold (ANTIQUOTE s, acc) = acc ^ s
in
List.foldl fold ""
end
end
end
end