Keep character width in unicode visual substitution

Here is a video^{[1](https://youtu.be/zQnExdDL63c)} demonstrating **Leksah** GHCJS support.
One interesting thing I see on this footage are unicode syntax substitutions for thing like `<-`, `=>` and etc. They have same width as original sequence, e.g. `⟵` arrow occupies double width of single character. Is it possible to hack Emacs unicode overlays to get similar behavior _(for all possible case `∷`,`⋙`, `≤`, `⟹` and etc.)_?