This expression catches an exception raised during evaluation of expression $⟨$exp$\mathrm{⟩}$. $⟨$match$\mathrm{⟩}$ is of the form

  $\mathrm{⟨}$pat${}_{\mathrm{1}}$$\mathrm{⟩}$ => $\mathrm{⟨}$exp${}_{\mathrm{1}}$$\mathrm{⟩}$
| $\mathrm{\cdots }$
| $\mathrm{⟨}$pat${}_n$$\mathrm{⟩}$ => $\mathrm{⟨}$exp${}_n$$\mathrm{⟩}$

that indicates the sequence of pairs of a exception pattern and expression to be evaluated if the raised exception is matched with the pattern.

Each pattern $⟨$pat${}_i$$\mathrm{⟩}$ must includes an exception constructor and therefore be of exception type exn. The type of every expression $⟨$exp${}_i$$\mathrm{⟩}$ must be identical to that of $⟨$exp$\mathrm{⟩}$. This expression is evaluated as follows.

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  $⟨$exp$\mathrm{⟩}$ is evaluated at first. If this evaluation is normally finished, its value is the value of this entire expression.

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  If an exception is raised during the evaluation, it tries to match the exception with each pattern from $⟨$pat${}_{\mathrm{1}}$$\mathrm{⟩}$ to $⟨$pat${}_n$$\mathrm{⟩}$. If it a match succeeds, it binds variables in $⟨$pat$\mathrm{⟩}$${}_i$ to corresponding parameters of the exception object and evaluates $⟨$exp${}_i$$\mathrm{⟩}$. The value of $⟨$exp${}_i$$\mathrm{⟩}$ is the value of this entire expression.

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  If the exception does not match with any pattern, the exception is propagated to the outer expression surrounding this expression.