Variable separation between items in a list environment

Question

Is it possible to define a list environment where each item has a user-specified amount of space below it? I'm looking for something to the effect of

\begin{myitemize}
    \item[1in] One
    \item[.5in] Two
    \item[2in] Three
\end{myitemize}

Some text...

In the compiled document, there should be 1 in of space between "One" and "Two", .5 in between "Two" and "Three", and 2 in between "Three" and "Some text".

My best attempt so far resembles

\documentclass{article}

\newlength{\itemspace}
\let\olditem\item

\newenvironment{myitemize}{
    \setlength{\itemspace}{0in}
    \begin{itemize}
    \renewcommand{\item}[1][0in]{
        \vspace{\itemspace}
        \olditem
        \setlength{\itemspace}{##1}}
}{
    \vspace{\itemspace}
    \end{itemize}
}


\begin{document}
 Some text
\begin{myitemize}
\item[0in] Foo\underline{\hspace{4.3in}}
\item Bar
\item Baz
\end{myitemize}
Some other text
\end{document}

The problem shows up in the first item. With 0in specified, the spacing between Foo and Bar should be the same as the spacing between Bar and Baz. However, I get something like this:

The problem seems to be that the length of the first line is exactly the maximum line width. Adding the vspace afterwards seems to be also add a small amount of horizontal space, forcing TeX to start a new line before the next item. Shortening the hspace to only 4.2in causes the extra line to disappear.

Edit: For an even simpler example of my problem,

\begin{itemize}
\item Foo\underline{\hspace{4.3in}}\vspace{0in}
\item Bar
\item Baz
\end{itemize}

has no extra line, but

\begin{itemize}
\item Foo\underline{\hspace{4.3in}}
\vspace{0in}
\item Bar
\item Baz
\end{itemize}

does. Why does this happen?

Answer 1

the bad spacing in your example is due to the spurious space you've added from the end of line characters, This just adds \par before \vspace so it is used in vertical mode, and % to avoid adding white space

\documentclass{article}

\newlength{\itemspace}
\let\olditem\item

\newenvironment{myitemize}{%
    \setlength{\itemspace}{0in}%
    \begin{itemize}%
    \renewcommand{\item}[1][0in]{%
        \par\vspace{\itemspace}%
        \olditem
        \setlength{\itemspace}{##1}}%
}{%
    \par\vspace{\itemspace}%
    \end{itemize}%
}


\begin{document}
 Some text
\begin{myitemize}
\item[0in] Foo\underline{\hspace{4.3in}}
\item Bar
\item Baz
\end{myitemize}
Some other text
\end{document}