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Proposal for improving pure situation, #3755

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Proposal for improving pure situation, to ensure that current MSL is …
HansOlsson Sep 30, 2025
c9e5041
Update chapters/functions.tex
HansOlsson Oct 16, 2025
7aedbf1
Update chapters/functions.tex
HansOlsson Oct 30, 2025
7db252a
Update chapters/functions.tex
HansOlsson Oct 30, 2025
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Update chapters/functions.tex
HansOlsson Nov 5, 2025
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11 changes: 4 additions & 7 deletions chapters/functions.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -225,16 +225,13 @@ \section{Pure Modelica Functions}\label{pure-modelica-functions}
\item
Functions defined in Modelica (non-external) are \emph{normally} assumed to be pure (the exception is the deprecated case below), if they are impure they shall be marked with the \lstinline!impure! keyword.
They can be explicitly marked as \lstinline!pure!\indexinline{pure}.
\begin{nonnormative}
However, since functions as default are pure it is not recommended to explicitly declare them as \lstinline!pure!.
\end{nonnormative}
\item
External functions must be explicitly declared with \lstinline!pure! or \lstinline!impure!.
\item
If a function is declared as \lstinline!impure! any function extending from it shall be declared as \lstinline!impure!.
\item
A deprecated semantics is that external functions (and functions defined in Modelica directly or indirectly calling them) without \lstinline!pure! or \lstinline!impure! keyword are assumed to be impure, but without any restriction on calling them.
Except for the function \lstinline!Modelica.Utilities.Streams.print!, a diagnostic must be given if called in a simulation model.
External functions not explicitly declared with \lstinline!pure! or \lstinline!impure! is deprecated.
\item
A deprecated semantics for functions without \lstinline!pure! or \lstinline!impure! keyword only applies in the following cases: external functions and functions defined in Modelica directly or indirectly (through functions without the \lstinline!pure! keyword) calling external functions without \lstinline!pure! keyword.
In these cases they are assumed to be impure, but without any restriction on calling them.
Comment on lines +233 to +234
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@henrikt-ma henrikt-ma Nov 5, 2025 •
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I think we are on the same page with what we want to achieve, but I am looking for a more clear way to describe it.

Let me give it another, concrete, try:

Suggested change
A deprecated semantics for functions without \lstinline!pure! or \lstinline!impure! keyword only applies in the following cases: external functions and functions defined in Modelica directly or indirectly (through functions without the \lstinline!pure! keyword) calling external functions without \lstinline!pure! keyword.
In these cases they are assumed to be impure, but without any restriction on calling them.
For a function without explicit purity, it is deprecated to call any function declared \lstinline!impure!, except when wrapped in \lstinline!pure($\ldots$)!.
\item
For a function not declared \lstinline!impure!, it is deprecated to call an external function not declared \lstinline!pure!, except when wrapped in \lstinline!pure($\ldots$)!.

Then, after the end{itemize}:

For purposes of symbolic transformations and optimizations, the deprecated semantics above imply that not only the functions explicitly declared \lstinline!impure! are the ones which cannot be treated as pure.
Instead, a function shall be treated as impure in the following cases (applied recursively):
\begin{itemize}
\item It is declared \lstinline!impure!.
\item It is an external function without explicit purity.
\item It calls another function treated as impure, except when wrapped in \lstinline!pure($\ldots$)!.
\end{itemize}

Edit: Slightly adjusted based on discussion in yesterday's language group meeting.

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@HansOlsson HansOlsson Nov 5, 2025

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The proposed line 235 contains an important case (one of the major issues in practice), which I think it should also be in the itemized list by changing the last item to: It calls a function assumed or declared impure, ....

\end{itemize}

Calls of pure functions used inside expression may be skipped if the resulting expression will not depend on the possible returned value; ignoring the possibility of the function generating an error.
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