Key values using l3keys (pencil symbol)

One optional argument is good, two can be remembered; more than two always require to look at the manual for their relative order.

Using a key-value syntax is much better when there are many options to set.

documentclass[letterpaper,11pt]{article}
usepackage{l3draw}

ExplSyntaxOn

% user interface
NewDocumentCommand{explpencil}{ O{} }
 {
  group_begin:
  aze_pencil_diagram:n { #1 }
  group_end:
 } 

% variables and variants
cs_generate_variant:Nn color_fill:n { V }

dim_new:N l__aze_pencil_size_dim
fp_new:N l__aze_pencil_linethickness_fp

keys_define:nn { aze/pencil }
 {
  size  .dim_set:N = l__aze_pencil_size_dim,
  color .tl_set:N  = l__aze_pencil_color_tl,
  color .initial:n = black,
  angle .fp_set:N  = l__aze_pencil_angle_fp,
  angle .initial:n = 110,
  thick .dim_set:N  = l__aze_pencil_thick_dim,
 }

% internal implementation
cs_new_protected:Nn aze_pencil_diagram:n
 {
  keys_set:nn { aze/pencil } { size=1ex, thick=0.08ex, #1 }
  draw_begin:
  draw_transform_rotate:n { l__aze_pencil_angle_fp } % angle
  draw_linewidth:n { l__aze_pencil_thick_dim }
  draw_cap_round:
  draw_join_round:
  draw_path_moveto:n { 0.000l__aze_pencil_size_dim , 0.125l__aze_pencil_size_dim }
  draw_path_lineto:n { 0.125l__aze_pencil_size_dim , 0.000l__aze_pencil_size_dim }
  draw_path_lineto:n { 1.000l__aze_pencil_size_dim , 0.000l__aze_pencil_size_dim }
  draw_path_lineto:n { 1.000l__aze_pencil_size_dim , 0.250l__aze_pencil_size_dim }
  draw_path_lineto:n { 0.125l__aze_pencil_size_dim , 0.250l__aze_pencil_size_dim }
  draw_path_close:
  color_fill:V l__aze_pencil_color_tl %color
  draw_path_use_clear:n { fill, stroke }
  draw_path_moveto:n { 1.125l__aze_pencil_size_dim , 0.0l__aze_pencil_size_dim }
  draw_path_lineto:n { 1.250l__aze_pencil_size_dim , 0.0l__aze_pencil_size_dim }
  draw_path_curveto:nn
    { 1.35l__aze_pencil_size_dim , 0.125l__aze_pencil_size_dim } 
    { 1.25l__aze_pencil_size_dim , 0.250l__aze_pencil_size_dim }
  draw_path_lineto:n { 1.125l__aze_pencil_size_dim , 0.25l__aze_pencil_size_dim }
  draw_path_close:
  color_fill:V l__aze_pencil_color_tl %color
  draw_path_use_clear:n { fill, stroke }
  draw_end:
 }

ExplSyntaxOff

begin{document}

explpencil{} sample text

explpencil[size=2ex] sample text

explpencil[size=1ex,color=red,angle=50,thick=0.01ex] sample text

end{document}

The possible options are size, color, angle and thick. The last one is mostly irrelevant for black pencils, it becomes important for colored ones.

Why do color and angle have initial values while size and thick don't? Because we'd like tha size to be in terms of ex, so that the drawing adapts to the context. If we set size initially to 1ex, this would be the dimension at the time the setting is done, so in the document preamble. Similarly for thick, so the values are set at call time, maybe to be overridden by an explicit option.