Catch

Evaluates an expression, recovering the value of any Throw inside it.

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

Catch(expr)$expr$'s value, or (if a Throw happened while evaluating it) the value that $Throw$ carried.

Catch(expr, tag)Like $Catch(expr)$, but only catches a $Throw$ whose tag matches $tag$ exactly — any other $Throw$ (untagged, or a different tag) propagates past this $Catch$.

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.
• An error that is not one of our own $Throw$s is never caught here — it propagates as a normal error would.

Examples

Basic #

with no Throw inside, Catch just returns the value

live ↗

In$\mathrm{Catch}(1+2)$Out$3$

Catch(expr, tag) recovers a Throw with a matching tag

live ↗

In$\mathrm{Catch}(\mathrm{Throw}(\mathrm{ok}, \mathrm{done}), \mathrm{done})$Out$\mathrm{ok}$

live ↗

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

Scope #

Throw exits the enclosing Catch, discarding the rest

live ↗

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

Only the first throw is reached

live ↗

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

A tag restricts which Catch receives the throw

live ↗

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

The inner Catch handles the throw, the outer list 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$458\,330$

About

Examples	10
Checked against	Wolfram Language 10 agree
Crosswalk	Wolfram Language

See also: Throw