One of the things we are not supposed to do on Chol Hamoed is writing. Nowadays most regular people do not write much on a daily basis anyway - we mostly type into computers and on smartphones, with little activity actually involving pen and paper. Growing up, before the advent of smartphones and when computers were not yet so common, writing was done a lot. Back then, if one had to writeon Chol Hamoed, either you just wouldn't or you would do it with your left hand when possible, thus employing a "shinuy".

People today wonder if that prohibition against writing on Chol Hamoed applies to the modern day version of writing - typing, using a computer and smartphone.

The question was posed to Rav Menachem Perl, Director of the Tzomet Institute.

Rav Perl paskened that one is allowed to type into a computer or smartphone despite the prohibition against writing on Chol Hamoed. Rav Perl explained that the prohibition is for writing with lasting material on lasting material - ink or graphite on paper.

Writing that does not last is allowed, as most melachot meant only for the short-term are allowed. Typing into computers is not a lasting act - each additional letter typed pushes over the letter to the next space, essentially erasing and rewriting it. Typing in a computer is similar to writing and achieves a similar result but it does not qualify as a melacha. Printing a document might be a problem, but typing on a computer is not.

Rav Perl qualified that this discussion is specific to the typing itself. If one is using the computer (or phone) to send messages of a business nature, that is a different problem as business should not normally be conducted on Chol Hamoed.
source: Kipa