You'd have a lot easier time if you made TRUE
and FALSE
two separate tokens.
%token TRUE FALSE NAND
%left NAND
%%
boolexp: boolexp NAND boolterm {$$ = !($1 && $3);}
| boolterm {$$=$1;}
;
boolterm: '(' boolexp ')' {$$ = $2;}
| bool {$$ = $1;}
;
bool: TRUE {$$ = 1;}
| FALSE ($$ = 0;}
;