Lib.try

STRUCTURE

SYNOPSIS

Apply a function and print any exceptions.

DESCRIPTION

The application try f x evaluates f x; if this evaluation raises an exception e, then e is examined and some information about it is printed before e is re-raised. If f x evaluates to y, then y is returned.

Often, a HOL_ERR exception can propagate all the way to the top level. Unfortunately, the information held in the exception is not then printed. try can often display this information.

FAILURE

When application of the first argument to the second raises an exception.

EXAMPLE

- mk_comb (T,F);
! Uncaught exception:
! HOL_ERR

- try mk_comb (T,F);

Exception raised at Term.mk_comb:
incompatible types
! Uncaught exception:
! HOL_ERR

Evaluation order can be significant. ML evaluates try M N by evaluating M (yielding f say) and N (yielding x say), and then f is applied to x. Any exceptions raised in the course of evaluating M or N will not be detected by try. In such cases it is better to use Raise. In the following example, the erroneous construction of an abstraction is not detected by try and the exception propagates all the way to the top level; however, Raise does handle the exception.

    - try mk_comb (T, mk_abs(T,T));
    ! Uncaught exception:
    ! HOL_ERR

    - mk_comb (T, mk_abs(T,T)) handle e => Raise e;

    Exception raised at Term.mk_abs:
    Bvar not a variable
    ! Uncaught exception:
    ! HOL_ERR

SEEALSO

Raise, trye