How to expand a value to place it in the ex command?

Question

The goal is to create a mapping that will:

• create a mapping for the user --> remap does the trick
• that gets the current expression under the cursor --> expand('<cexpr>') will do the trick
• put this into the command line, so the user can adjust as necessary

What I've tried so far:

• :let exp = expand('<cexpr>') -> gets the value, for example "my_expr"
• nnoremap = :Evaluate<Space> expand('<cexpr>') -> writes that text in there instead of resolving it
• nnoremap <expr> = 'Evaluate '. expand('<cexpr>') . ' = ' --> changes to visual mode

should pre-set the ex command to :Evaluate my_expr so the user can adjust the expression before pressing enter.

Answer 1

I think you're just missing a colon before Evaluate...

nnoremap <expr> = ':Evaluate '. expand('<cexpr>') . ' = '

When I try it with the colon I get

Evaluate foo = 

...waiting at the command line.

Without the colon I get Visual mode just like you describe. Remember, this is a Normal mode mapping so it's like you literally enter E followed by v in Normal mode which places you in Visual mode. Adding the colon makes sure you go to the command line first.