Forest for linguistics - proportional alignment like in qtree

Question

I'm new to forest. Is there a style (or a modification of what's below) which produces the alignment of empty terminal nodes F3 and F4 in Tree A just like in Tree B?

\documentclass{standalone}
\usepackage{forest}
%\forestset{nice nodes/.style={for tree={inner sep=0pt,s sep=.2in,align=center, anchor=north}},default preamble=nice nodes, }
\useforestlibrary{linguistics}
\forestapplylibrarydefaults{linguistics}
\begin{document}

Tree A:

\begin{forest}
[
[DP, name=target[a labelled\\ subtree, roof]]  
[F$_4$P[F$_4$]
[F$_3$P[F$_3$]
[F$_2$P[F$_2$][\hspace{10pt}F$_1$P $\Rightarrow$ something, 
name=t[is there a way\\to move this label\\to the right?, roof]]]]]]]
\end{forest}

Tree B:

\begin{forest}
[
[DP, name=target[a labelled\\ subtree, roof]]  
[F$_4$P[F$_4$[$\emptyset$]]
[F$_3$P[F$_3$[]]
[F$_2$P[F$_2$][\hspace{10pt}F$_1$P $\Rightarrow$ something, 
name=t[I miss qtree, roof]]]]]]]
\end{forest}
\end{document}

A separate issue is the problem of moving a node label (F4P in the trees) to the right, so that it is not centered wrt to the node. Here is what I get:

PS. The skeleton I'm using tolerates qtree and tikz-qtree but does not want to tolerate arrows in neither qtree nor tikz-qtree in XeLaTeX, so I'll need to stick to forest, I guess. That's why I'm trying to have a nice proportional tree in forest rather than using simply qtree.

Answer 1

I think for your main question, you probably just want to add fit=band to the tree.

Note that anchor=center is default for linguistics.

\documentclass[border=10pt,multi,tikz]{standalone}
\usepackage[linguistics]{forest}
\forestset{
  nice nodes/.style={
    for tree={
      inner sep=0pt,
      fit=band,
    },
  },
  default preamble=nice nodes,
}
\begin{document}
\begin{forest}
  [
    [DP, name=target
      [nodes F$_3$ and F$_4$ do\\
      align appropriately\\
      (I think)\\
      wrt F$_2$, roof
      ]
    ]
    [F$_4$P
      [F$_4$]
      [F$_3$P
        [F$_3$]
        [F$_2$P
          [F$_2$]
          [\hspace{10pt}F$_1$P$\Rightarrow$something, name=t
            [is there a way\\to move this label\\to the right?, roof
            ]
          ]
        ]
      ]
    ]
  ]
\end{forest}
\end{document}

I'm not quite sure what you want to shift where. What you are calling the node label is actually not a label, but the node content. Nodes can have labels and contents, so this may be a source of confusion.

You can shift the node at the bottom right right (or left). However, I'm not sure if that is really what you mean.

            [is there a way\\to move this label\\to the right?, roof, xshift=10pt
            ]

To avoid a lopsided roof, we can redefine the edge path.

            [is there a way\\to move this label\\to the right?, xshift=10pt, edge path'={(!u.parent anchor) ++(5pt,0) -- (.north east) -- (.north west) -- cycle}
            ]

If you need this a lot, a style would be convenient.

  shift with roof/.style={
    xshift=#1,
    edge path'={(!u.parent anchor) ++(#1/2,0) -- (.north east) -- (.north west) -- cycle}
  },

Then we can write just

            [is there a way\\to move this label\\to the right?, shift with roof=10pt
            ]

to produce the same output as above.

You can make the top less 'pointy' by increasing the distance between the root's children. Possibly a bit extreme:

  [, s sep'+=20pt

A bit less extreme may be better.

\documentclass[border=10pt,multi,tikz]{standalone}
\usepackage[linguistics]{forest}
\forestset{
  nice nodes/.style={
    for tree={
      inner sep=0pt,
      fit=band,
    },
  },
  default preamble=nice nodes,
  shift with roof/.style={
    xshift=#1,
    edge path'={(!u.parent anchor) ++(#1/2,0) -- (.north east) -- (.north west) -- cycle}
  },
}
\begin{document}
\begin{forest}
  [, s sep'+=10pt
    [DP, name=target
      [nodes F$_3$ and F$_4$ do\\
      align appropriately\\
      (I think)\\
      wrt F$_2$, roof
      ]
    ]
    [F$_4$P
      [F$_4$]
      [F$_3$P
        [F$_3$]
        [F$_2$P
          [F$_2$]
          [\hspace{10pt}F$_1$P$\Rightarrow$something, name=t
            [is there a way\\to move this label\\to the right?, shift with roof=10pt
            ]
          ]
        ]
      ]
    ]
  ]
\end{forest}
\end{document}

Answer 2

\documentclass{standalone}
\usepackage{forest}
\forestset{
  nice nodes/.style={
    for tree={
      inner sep=1pt, s sep=12pt,
      fit=band,
    },
  },
  default preamble=nice nodes,
}
\useforestlibrary{linguistics}
\forestapplylibrarydefaults{linguistics}
\begin{document}

\begin{forest}
 [
 [DP, name=target[nodes F$_3$ and F$_4$ do\\ not align vertically\\ wrt           F$_2$, roof]]  
 [F$_4$P[F$_4$]    [F$_3$P[F$_3$]
  [F$_2$P[F$_2$]    [F$_1$P, name=t[I miss qtree, roof]] 
{ \draw (.east) node[right]{$\Rightarrow$ \textit{adjective}}; } ]]]]]
   %\draw[->] (t) to[out=south west,in=south] (target);
\end{forest}
\end{document}

Which give me this: