I've just been playing with SMath Studio (Android) for an hour or so - it seems to me that the input syntax / parsing is exactly the same as the browser version "SMath Studio in the Cloud"
and I assume they use a lot of the same code.
Typing a colon automatically generates a displayed := waiting for you to enter your expression. Typing a colon followed by an equals sign places a second trailing equals sign which is where the "in-line evaluation" of your expression appears.
For example typing the following sequence of characters (to assign a length of 5 inches to variable x) without typing an equals symbol
x:5'in
displays
x := 5 in
Typing the following sequence of characters (to assign a length of 5 inches to variable x, and evaluate it in default units - in my case metric units), by typing an equals symbol (BEFORE or after entering the expression)
x:=5'in
displays
x := 5 in = 0.127 m
I haven't been able to find any on-line documentation, and a lot of menu options seem to be missing (e.g. modifying display formats etc), so you might need to send your file to the desktop version or browser version to access those features. It's also possible that a "full-featured" SMath worksheet created in the desktop version might not work properly in the mobile version, but I haven't tested this. Even so, even with its limited feature set, it'll be a very useful tool for me - I'll load up a few of my common calculation worksheets onto my phone for doing quick checks in the field. (I certainly can't complain about the value for money!)
I did notice that at the bottom right of the screen, there is a double-button. The left button (default) opens a basic menu of options, and the right button opens a translucent custom keyboard, with quick access to common mathematical operators, units, etc. It takes up quite a bit of screen "real estate" on my phone (although you can see the underlying worksheet through it), but it works very well on my 10" tablet.