Flycheck has a symbol that tells in which line an error or warning have been encountered. I have seen spacemacs has its flycheck symbol changed. Default symbol is a double arrow; while spacemacs's symbol is a dot. I can't find a function to change it. How did spacemacs do that?
After reading flycheck source code..
1) You must define a fringe bitmap (the "symbol") with 'define-fringe-bitmap' function.
2) Use 'flycheck-define-error-level' function to load the new bitman
3) Enable Flycheck mode
Example:
(define-fringe-bitmap 'flycheck-fringe-bitmap-ball
(vector #b00000000
#b00000000
#b00000000
#b00000000
#b00000000
#b00111000
#b01111100
#b11111110
#b11111110
#b01111100
#b00111000
#b00000000
#b00000000
#b00000000
#b00000000
#b00000000
#b00000000)))
(flycheck-define-error-level 'error
:severity 100
:compilation-level 2
:overlay-category 'flycheck-error-overlay
:fringe-bitmap 'flycheck-fringe-bitmap-ball
:fringe-face 'flycheck-fringe-error
:error-list-face 'flycheck-error-list-error)