\documentclass{article}

%include lhs2TeX.fmt
%include lhs2TeX.sty
%include polycode.fmt

\newenvironment{calculation}{%
  \renewcommand\commentbegin{\qquad\{\ }%
  \renewcommand\commentend{\}}%
  \setlength{\blanklineskip}{1ex}%
  \renewcommand\hsindent[1]{\qquad}}{}
  
\begin{document}
\begin{calculation}
\begin{spec}
    map (+1) [1,2,3]

==    {- desugaring of |(:)| -}

    map (+1) (1 : [2,3])

==    {- definition of |map| -}

    (+1) 1  :  map (+1) [2,3]

==    {- performing the addition on the head -}

    2       :  map (+1) [2,3]

==    {- recursive application of |map| -}

    2       :  [3,4]

==    {- list syntactic sugar -}

    [2,3,4]
\end{spec}
\end{calculation}

\end{document}