longtable width

I am using longtable and want to specify the width of a column, however this does not work for me, first column need to be 2 cm for instance:

\documentclass[11pt,a4paper]{report}
\usepackage{longtable}
\begin{document}
\begin{longtable}{l{2cm}l}
A & B
\end{longtable}
\end{document}

• p{2cm} or define a new columntype using the array package – user31729 Feb 20 '18 at 20:28
• A longtable environment is always centered.... so flushing to the side could be a little difficult. You can use @p{2cm} to get at least rid of some small spacing at the left margin. And please, don't change the question in comments... – user31729 Feb 20 '18 at 20:37
• Sorry, I meant @{}p{2cm} of course... – user31729 Feb 20 '18 at 20:45
• \begin{longtable}[l]{@{}p{2cm}l@{}} – David Carlisle Feb 20 '18 at 21:47
• @DavidCarlisle Well, let's assume I just forgot about [l] ;-) – user31729 Feb 20 '18 at 21:52

\begin{longtable}[l]{@{}p{2cm}l@{}}

to left align the table ([l]), have no left border space in the first column {@{}), and set the first column in a parbox of width 2cm (p{2cm}).