Form validation, specifically verifying user input, is a common requirement in GUI applications. With statically typed languages it can get a bit complicated. In this post I’ll introduce form decoding, a form validation method specifically suited to statically typed languages. I’ll also introduce elm-form-decoder, an implementation of the concept in Elm. Sample Application Say we have a social networking application for goats. For our purposes it doesn’t matter how they use keyboards with their two-fingered hands.