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Wildcard placeholders

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JanUlrich 2024-04-04 14:38:16 +02:00
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unify.tex
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@ -110,6 +110,20 @@ gets the same wildcard twice.
% Free variables are not allowed to leave their scope.
% This is ensured by type variables which are not allowed to be assigned type holding free variables.
\textbf{Wildcard Placeholders:}
The vital part are the \rulename{Subst} and \rulename{Normalize} rules.
They assert that a normal type placeholder is never replaced by a type containing free variables.
\rulename{Normalize} replaces Wildcard placeholders with normal placeholders right before they get substituted by \rulename{Subst}.
The idea is to keep the possibility of replacing a wildcard placeholder with a free variable as long as possible.
As soon as they appear in a $\ntv{a} \doteq \type{T}$ constraint they can no longer be replaced by free variables.
A type solution for a normal type placeholder will never contain free variables.
This is needed to guarantee well-formed type solutions and also keep free variables inside their scope.
\begin{lstlisting}
let y = { let x = v in v.get() } in y.get()
\end{lstlisting} %TODO: explain: here y has to be a type without free variables.
\subsection{Algorithm}
\newcommand{\gtype}[1]{\type{#1}}