On the behavior of the enumerate environment

Question

I have defined a very simple custom environment for homework problems, which is basically just enumerate but without the indents:
newcounter{hwprob}
newenvironment{hwprob}{refstepcounter{hwprob} textbf{thehwprob.} ~ }{}

If I open an actual enumerate environment inside hwprob, this happens:
begin{hwprob}
   begin{enumerate}
      item Some text.
      item Some other text.
   end{enumerate}
end{hwprob}

I'm not sure why the first item 1. does not start on the same line as 1. I tried removing the tilde, but to no avail. What puzzles me is that I know that when using enumerate inside the proof environment the first item will be in the same line as Proof., with the correct indentation and all.
What can I do to fix this? Is this a result of how I defined hwprob, or is it inherent to enumerate?

muzimuzhi Z · Accepted Answer

Here is the "how" part: define hwprob as a list environment, and auto insert an item at the beginning of hwprob.
documentclass{article}
usepackage{enumitem}

newlist{hwprob}{enumerate}{1}
setlist[hwprob]{label=textbf{arabic*.}, first*=item}

begin{document}
begin{hwprob}
   begin{enumerate}
      item Some text.
      item Some other text.
   end{enumerate}
end{hwprob}
end{document}

For the "why" part, I guess it has something to do with trivlist (perhaps with parshape), which is the common base used by both list and theorem environments.

Mico · Answer

The formatting you wish to achieve -- have the first item in the enumerate list start on the current line rather than start after a line break -- happens if the enumerate environment is started inside one LaTeX's list or trivlist environments.
To achieve your formatting objective, I suggest you load the enumitem package and use its newlist and setlist macros to create a bespoke enumerate-like list environment called hwprob.

documentclass{article}
usepackage{enumitem}
newlist{hwprob}{enumerate}{1}
setlist[hwprob,1]{label=bfseriesarabic*.,left=0pt}

begin{document}
begin{hwprob}
item
   begin{enumerate}
      item Some text.
      item Some other text.
   end{enumerate}
end{hwprob}
end{document}

On the behavior of the enumerate environment

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