Terminate on error +f,-f

Terminate on error +f,-f

In normal use, the evaluation of an expression is abandoned completely if a run-time error occurs, such as a failed pattern match or an attempt to divide by zero. For example:

 Prelude> [1 `div` 0]
 [
 Program error: {primDivInt 1 0}

 Prelude> [1 `div` 0, 2]
 [
 Program error: {primDivInt 1 0}

 Prelude>

This is often useful during program development because it means that errors are detected as soon as they occur. However, technically speaking, the two expressions above have different meanings; the first is a singleton list, while the second has two elements. Unfortunately, the output produced by Hugs does not allow us to distinguish between the values.

The -f option can be used to make the Hugs printing option a little more accurate; this should normally be combined with -u because the built-in printer is better than the user-defined show functions at recovering from evaluation errors. With these settings, if the interpreter encounters an irreducible subexpression, then it prints the expression between a matching pair of braces and attempts to continue with the evaluation of other parts of the original expression. For the examples above, we get:

 Prelude> :set -u -f
 Prelude> [1 `div` 0]       -- value is [bottom]
 [{primDivInt 1 0}]
 Prelude> [1 `div` 0, 2]
 [{primDivInt 1 0}, 2]      -- value is [bottom, 2]
 Prelude>

Reading an expression in braces as bottom, the output produced here shows the correct values, according to the semantics of Haskell. Of course, it is not possible to detect all occurrences of bottom like this, such as those produced by a nonterminating computation:

 Prelude> last [1..]
 ^C{Interrupted!}      -- nothing printed until interrupted

 Prelude>

Note that the basic method of evaluation is the same with both the +f and -f options; all that changes is the way that the printing mechanism deals with certain kinds of runtime error.