PageWidth not inserting line breaks into long strings

Question

I am having some trouble setting the width of my output. I have some very long expressions, and I would like to output them as strings of maximum length 80 characters. For example, I have a very long expression "beta" and I would like to output to a file "RGEs_nicks.txt". So I have tried

streamM = OpenWrite["RGEs_nicks.txt", PageWidth -> 80];
WriteString[streamM, ToString[beta // InputForm]];
Close[streamM];

However, this is not putting in any line breaks, and the output is just one very long line. How do I get this to be formatted output? It works fine if I use just Write instead of WriteString, but I want to be able to add some other tricky stuff before and after so I need the WriteString. Any clues would be much appreciated!

Answer 1

Assuming you really need to use WriteString for some reason, you could use ExportString, which respects PageWidth, to do the formatting for you:

streamM = OpenWrite["RGEs_nicks.txt"];
WriteString[streamM, ExportString[beta, "Package", PageWidth -> 80]];
Close[streamM];

The advantage of using ExportString is that it knows how to preserve a valid syntax when doing the line wrapping, as opposed to blind string splitting.