Throw

Aborts evaluation up to the nearest enclosing Catch, carrying a value.

✓ checked against that system · ! checked, with disagreements (hover a chip)Wolfram LanguageThrow✓↗

Throw(value)Unwinds evaluation up to the nearest enclosing Catch, which returns $value$.

Throw(value, tag)Like $Throw(value)$, but only a $Catch(expr, tag)$ matching $tag$ exactly stops it — an untagged or non-matching $Catch$ lets it propagate further out.

Typed signatures

Type	Package	Applies
(any, any?) -> any	combinatorics	always

Domain: Collections

Details

• Only exact tag equality is matched, not Wolfram's fuller pattern-matching form.
• A $Throw$ that reaches no matching $Catch$ propagates out of the whole evaluation.

Examples

Basic #

Throw aborts evaluation, Catch recovers the value

live ↗

In$\mathrm{Catch}(\mathrm{Throw}(\mathrm{abandoned}))$Out$\mathrm{abandoned}$

live ↗

In$\mathrm{Catch}(\mathrm{Do}(\mathrm{If}(k!\gt10^{10}, \mathrm{Throw}(k)), \bigl\lbrack k, 100\bigr\rbrack))$Out$14$

Scope #

a tag mismatch propagates past a non-matching Catch to one that matches

live ↗

In$\mathrm{Catch}(\mathrm{Catch}(\mathrm{Throw}(\mathrm{deep}, x), y), x)$Out$\mathrm{deep}$

The thrown value escapes the list and is returned directly

live ↗

In$\mathrm{Catch}(\bigl\lbrack a, \mathrm{Throw}(b), c\bigr\rbrack)$Out$b$

Evaluation stops at the first Throw, so only $a$ comes back

live ↗

In$\mathrm{Catch}(\bigl\lbrack\mathrm{Throw}(a), \mathrm{Throw}(b), \mathrm{Throw}(c)\bigr\rbrack)$Out$a$

A tagged throw is caught by a Catch with the matching tag

live ↗

In$\mathrm{Catch}(\mathrm{Throw}(a, u), u)$Out$a$

An inner Catch absorbs the throw, and the outer evaluation continues

live ↗

In$\mathrm{Catch}(\bigl\lbrack\mathrm{Catch}(\bigl\lbrack a, \mathrm{Throw}(b), c\bigr\rbrack), d, k\bigr\rbrack)$Out$\bigl\lbrack b, d, k\bigr\rbrack$

Applications #

live ↗

In$\mathrm{Catch}(\mathrm{Do}(\mathrm{If}(\mathrm{IsPrime}(k), \mathrm{Throw}(k)), \bigl\lbrack k, 10^{10}, 10^{10}+1\,000\bigr\rbrack))$Out$10\,000\,000\,019$

live ↗

In$\mathrm{Catch}(\mathrm{Do}(\mathrm{If}(17^{k}\bmod19=1, \mathrm{Throw}(k)), \bigl\lbrack k, 19\bigr\rbrack))$Out$9$

live ↗

In$\mathrm{Catch}(\mathrm{NestList}(\operatorname{\_1}↦\text{if }\operatorname{\_1}\gt1\,000\text{ then }\mathrm{Throw}(\operatorname{\_1})\text{ else }\operatorname{\_1}^2+1, 2, 6))$Out$458\,330$

Possible issues #

live ↗

In$\mathrm{Catch}(\mathrm{NestList}(\operatorname{\_1}↦\text{if }\operatorname{\_1}\gt1\,000\text{ then }\mathrm{Throw}(\operatorname{\_1})\text{ else }\operatorname{\_1}^2+1, 1, 7))$Out$458\,330$

About

Examples	11
Checked against	Wolfram Language 11 agree
Crosswalk	Wolfram Language

See also: Catch