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Unitful literals #3688

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7694426
Fix misuses of UNSIGNED-NUMBER where an integer is expected
henrikt-ma May 7, 2025
2934636
Introduce syntax for attaching unit to a numeric literal
henrikt-ma May 7, 2025
d4c7b3c
Add extended example
henrikt-ma Sep 13, 2025
27b5c74
Fix 'ambiguity' → 'ambiguously'
henrikt-ma Sep 29, 2025
f2f7838
Make space before unit non-breaking
henrikt-ma Oct 14, 2025
33e933e
State that unitful literals of temperature are absolute quantities
henrikt-ma Oct 14, 2025
04261cd
Revert "State that unitful literals of temperature are absolute quant…
henrikt-ma Oct 20, 2025
df98a77
Allow unitful temperature literals to take absoluteValue from context
henrikt-ma Oct 20, 2025
c2f5daf
Require that unit inconsistency can be detected
henrikt-ma Oct 20, 2025
b14d590
Be careful not to speak of "error" since unit checking is not defined
henrikt-ma Oct 20, 2025
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6 changes: 3 additions & 3 deletions chapters/functions.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -1093,13 +1093,13 @@ \section{Derivatives and Inverses of Functions}\label{derivatives-and-inverses-o

\begin{annotationdefinition}[smoothOrder]
\begin{synopsis}[grammar]\begin{lstlisting}
"smoothOrder" "=" UNSIGNED-NUMBER
"smoothOrder" "=" UNSIGNED-INTEGER
"smoothOrder"
"("
"normallyConstant" "=" IDENT
{ "," "normallyConstant" "=" IDENT }
")"
"=" UNSIGNED-NUMBER
"=" UNSIGNED-INTEGER
\end{lstlisting}\end{synopsis}
\begin{semantics}
This annotation has only an effect within a function declaration.
Expand Down Expand Up @@ -1137,7 +1137,7 @@ \section{Derivatives and Inverses of Functions}\label{derivatives-and-inverses-o
"(" derivative-constraint { "," derivative-constraint } ")"

derivative-constraint :
"order" = UNSIGNED-NUMBER
"order" = UNSIGNED-INTEGER
| "noDerivative" = IDENT
| "zeroDerivative" = IDENT
\end{lstlisting}\end{synopsis}
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9 changes: 6 additions & 3 deletions chapters/syntax.tex
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Expand Up @@ -394,7 +394,7 @@ \subsection{Expressions}\label{expressions1}
primary [ ( "^" | ".^" ) primary ]

primary :
UNSIGNED-NUMBER
numeric
| STRING
| false
| true
Expand All @@ -405,8 +405,11 @@ \subsection{Expressions}\label{expressions1}
| "{" array-arguments "}"
| end

UNSIGNED-NUMBER :
UNSIGNED-INTEGER | UNSIGNED-REAL
numeric :
( UNSIGNED-INTEGER | UNSIGNED-REAL ) [ unit-of-measurement ]

unit-of-measurement :
Q-IDENT

type-specifier :
["."] name
Expand Down
31 changes: 31 additions & 0 deletions chapters/unitexpressions.tex
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Expand Up @@ -104,3 +104,34 @@ \section{The Syntax of Unit Expressions}\label{the-syntax-of-unit-expressions}

The unit expression \lstinline!"T"! means tesla, but note that the letter \lstinline!T! is also the symbol for the prefix tera which has a multiplier value of 10\textsuperscript{12}.
\end{example}


\section{Unitful Literals}

The \lstinline[language=grammar]!numeric! in the grammar allows a unit to be attached to a numeric value.
In this case, the unit is given as a \lstinline[language=grammar]!Q-IDENT! rather than a string, since the use of a string could be ambiguity interpreted as \lstinline[language=grammar]!description-string! in some contexts.

\begin{example}
An acceleration of 9.8 meters per square second can be expressed as \lstinline!9.8'm/s2'!.
\end{example}

\begin{example}
Adding 2~centimeters to a length.
A user has set \lstinline!x! to 10~centimeters with the use of \lstinline!displayUnit!, and intends to make \lstinline!y! 2~centimeters bigger than \lstinline!x!:
\begin{lstlisting}[language=modelica]
parameter Real x(unit = "m", displayUnit = "cm") = 0.1;
equation
y = x + 2; /* Intention: Add 2 centimeters to x. */
\end{lstlisting}
The result is that 2~meters instead of 2~centimeters is being added to \lstinline!x!.

Using a unitful literal makes the intention clear:
\begin{lstlisting}[language=modelica]
y = x + 2'cm';
\end{lstlisting}
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@HansOlsson HansOlsson Sep 30, 2025

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Suggested change
\end{lstlisting}
\end{lstlisting}
Tools must infer the unit information of the unitful literal, including absolueValue when applicable, from the rest of the model, and verify that the given unit is compatible.
It is an error if the needed unit information cannot be inferred.
Diagnostics may be given for consistent units that shouldn't be mixed (like \lstinline!"N.m"! and \lstinline!"J"!).
Special care is needed to handle rotational frequencies.

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@HansOlsson HansOlsson Sep 30, 2025

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To make it clearer that it is just adding lines to clarify, and that the following lines are unchanged.

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@henrikt-ma henrikt-ma Oct 20, 2025

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I am aiming for a rule that doesn't forbid this:

Real pressure(unit = "Pa") = 5'N' / 20'm2';

So instead of speaking of inferring the unit of the unitful literal from the context, I am requiring that unit consistency can be verified.

I find saying something about this model outside the scope of this PR:

Real force(unit = "N") = 3;
Real distance(unit = "m") = 5;
Real work(unit = "J") = force * distance;

Similarly, I find statements regarding rotational frequencies to be a separate topic.

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@HansOlsson HansOlsson Oct 23, 2025

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It wasn't for the case above, or even for:

Real force(unit = "N") = 3;
Real distance(unit = "m") = 5;
Real lever(unit="m")=2;
Real work(unit = "J") = force * distance;
Real torque(unit="N.m") = force * lever;

but to allow diagnostics for the subtly different:

Real torque2(unit="N.m") = work;

Similarly, for rotational frequencies.

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@HansOlsson HansOlsson Oct 30, 2025 •
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Or more precisely to give strong warnings for:

Real torque(unit="N.m") = 12 'J';
Real work(unit="J")= 20 'N.m';

And rotational frequencies are still unresolved.

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@henrikt-ma henrikt-ma Dec 1, 2025 •
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Or more precisely to give strong warnings for:

Real torque(unit="N.m") = 12 'J';
Real work(unit="J")= 20 'N.m';

I would expect the above to equally valid as when using a helper variable:

constant Real(unit = "J") x = 12;
Real torque(unit="N.m") = x;
constant Real y(unit = "N.m") = 20;
Real work(unit="J")= 20 y;

What sort of warnings that should be given for the above is a topic for unit checking, not a topic for this PR.

And rotational frequencies are still unresolved.

Just as with the "J" vs "N.m", one can run into the same problems when using helper variables instead of unitful literals, so I don't see why we would need to solve this messy problem as part of this PR.

It is currently not expected that tools will perform an implicit unit conversion to make the addition possible, in which case the addition should be rejected as a unit inconsistency.
The unit inconsistency can then be resolved by using a unit compatible with the unit of \lstinline!x!:
\begin{lstlisting}[language=modelica]
y = x + 0.02'm';
\end{lstlisting}
\end{example}