Already defined or Mismatched input ':=' expecting '=' errors

Question

I am trying to plot this indicator but I get the "already defined" error for "=" or "Mismatched input ':=' expecting '='" for ":=" when I try assigning to tuple

    lb( x, a0, a1, a2, a3, c ) =>
        if( x == 1 and c > a3 )
            [x, a1, a2, a3, c]
        else if( x == 1 and c < a0 )
            [-1, a3, a3, a2, c]
        else if( x == -1 and c < a3 )
            [x, a1, a2, a3, c]
        else if( x == -1 and c > a0 )
            [ 1, a3, a3, a2, c ]
            

    f_x  = 1
    f_a0 = 0.0
    f_a1 = 0.0
    f_a2 = 0.0
    f_a3 = 0.0

    [f_x, f_a0, f_a1, f_a2, f_a3] := lb( f_x, f_a0, f_a1, f_a2, f_a3, close ) 


    plot( f_a0 , color=color.red ) 

Thank you!

Answer 1

You cannot use := to assign values to a tuple.
Only = is allowed, which means you must not define those target tuple variables on beforehand.

Something like this would work though:

//@version=5
indicator("My Script", overlay=true)

lb( x, a0, a1, a2, a3, c ) =>
    if( x == 1 and c > a3 )
        [x, a1, a2, a3, c]
    else if( x == 1 and c < a0 )
        [-1, a3, a3, a2, c]
    else if( x == -1 and c < a3 )
        [x, a1, a2, a3, c]
    else if( x == -1 and c > a0 )
        [ 1, a3, a3, a2, c ]

var float f_x = na
var float f_a0 = na
var float f_a1 = na
var float f_a2 = na
var float f_a3 = na

if barstate.isfirst
    f_x  := 1
    f_a0 := 0.0
    f_a1 := 0.0
    f_a2 := 0.0
    f_a3 := 0.0

[f_x_dummy, f_a0_dummy, f_a1_dummy, f_a2_dummy, f_a3_dummy] = lb( f_x, f_a0, f_a1, f_a2, f_a3, close ) 

f_x  := f_x_dummy
f_a0 := f_a0_dummy
f_a1 := f_a1_dummy
f_a2 := f_a2_dummy
f_a3 := f_a3_dummy

plot( f_a0 , color=color.red )