\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\tikzset{->-/.style 2 args = {decoration={markings, mark=at position{#1} with {\arrow#2}},
                              postaction=decorate}}
\begin{document}

\verb+https://lists.tug.org/pgf-tikz+  Mail to \verb+pgf-tikz@tug.org+

\begin {tikzpicture}
\draw [->- = {0.6} {latex}]  (0,0) -- (1,0) ;
\draw [->- = {0.6}{{latex}}] (2,0) -- (3,0) ;

%% Pb with this form (no space between arguments)
%% \draw [->- = {0.6}{latex}] (4,0) -- (5,0) ; 
%% ! Package pgf Error: Unknown arrow tip kind 'l'.
%% See the pgf package documentation for explanation.
%% Type  H <return>  for immediate help.
%% ...                                              
%% l.17 \draw [->- = {0.6}{latex}]   (4,0) -- (5,0) ;


\draw [->-= {0.6}{[ultra thick]{>}}] (6,0) -- (7,0) ;

%% Pb with this form (a space between arguments)
%% \draw [->-= {0.6} {[ultra thick]{>}}] (8,0) -- (9,0) ;
%% ! Package pgfkeys Error: I do not know the key 'ultra thick' and I am going to 
%% ignore it. Perhaps you misspelled it.
%%
%% See the pgfkeys package documentation for explanation.
%% Type  H <return>  for immediate help.
%% ...                                              
%% l.28 ...{0.6} {[ultra thick]{>}}] (8,0) -- (9,0) ;
\end {tikzpicture}
\end {document}
