I usually use only shampoo to wash my hairs becasue once time i have used a soap to wash my hairs and my hairs lost natural shine. After this, i have never used soap on my hair.
I have used soap to wash my hair many times before especially when I'm at my village with other family members. It is far away from the city or any shops. I never had much problem apart from the trouble of combing my hair after that. I think it's okay if we apply hair conditioner after washing our hair with the soap. And also leave the hair until it's completely dry before combing it.
Yes, I've tried using a bar soap to clean my hair when I was a kid. The result was awful. My hair was sticky and a bit difficult to comb. I also use bar soap for my hair when I need to remove hair spray. Back in the 80s when all girls use tons of hair spray for teased hair, using soap was the reliable way of removing all that gunk from my hair. Of course, I still had to use shampoo and conditioner after rinsing the soap to avoid the sticky feel. Nowadays, there are liquid soaps and shower gels that can also double as a shampoo. I still don't prefer using these products on my hair unless it's the only choice I have, like when I'm traveling and I forgot to bring my own shampoo.
You shouldn't use any regular old soap for washing hair. I have tried it with ivory soap before and it just made my hair dry and thick. I have been washing my hair with olive oil soap for about a month and my hair seems perfectly normal. I read that it's okay to use for hair too. There are certain kinds of soap meant for hair and I guess you can save money this way but, special soap is probably more expensive.