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<ol class="chapter"><li class="affix"><a href="index.html">Introduction</a></li><li><a href="hello.html"><strong aria-hidden="true">1.</strong> Hello World</a></li><li><ol class="section"><li><a href="hello/comment.html"><strong aria-hidden="true">1.1.</strong> Comments</a></li><li><a href="hello/print.html"><strong aria-hidden="true">1.2.</strong> Formatted print</a></li><li><ol class="section"><li><a href="hello/print/print_debug.html"><strong aria-hidden="true">1.2.1.</strong> Debug</a></li><li><a href="hello/print/print_display.html"><strong aria-hidden="true">1.2.2.</strong> Display</a></li><li><ol class="section"><li><a href="hello/print/print_display/testcase_list.html"><strong aria-hidden="true">1.2.2.1.</strong> Testcase: List</a></li></ol></li><li><a href="hello/print/fmt.html"><strong aria-hidden="true">1.2.3.</strong> Formatting</a></li></ol></li></ol></li><li><a href="primitives.html"><strong aria-hidden="true">2.</strong> Primitives</a></li><li><ol class="section"><li><a href="primitives/literals.html"><strong aria-hidden="true">2.1.</strong> Literals and operators</a></li><li><a href="primitives/tuples.html"><strong aria-hidden="true">2.2.</strong> Tuples</a></li><li><a href="primitives/array.html"><strong aria-hidden="true">2.3.</strong> Arrays and Slices</a></li></ol></li><li><a href="custom_types.html"><strong aria-hidden="true">3.</strong> Custom Types</a></li><li><ol class="section"><li><a href="custom_types/structs.html"><strong aria-hidden="true">3.1.</strong> Structures</a></li><li><a href="custom_types/enum.html"><strong aria-hidden="true">3.2.</strong> Enums</a></li><li><ol class="section"><li><a href="custom_types/enum/enum_use.html"><strong aria-hidden="true">3.2.1.</strong> use</a></li><li><a href="custom_types/enum/c_like.html"><strong aria-hidden="true">3.2.2.</strong> C-like</a></li><li><a href="custom_types/enum/testcase_linked_list.html"><strong aria-hidden="true">3.2.3.</strong> Testcase: linked-list</a></li></ol></li><li><a href="custom_types/constants.html"><strong aria-hidden="true">3.3.</strong> constants</a></li></ol></li><li><a href="variable_bindings.html"><strong aria-hidden="true">4.</strong> Variable Bindings</a></li><li><ol class="section"><li><a href="variable_bindings/mut.html"><strong aria-hidden="true">4.1.</strong> Mutability</a></li><li><a href="variable_bindings/scope.html"><strong aria-hidden="true">4.2.</strong> Scope and Shadowing</a></li><li><a href="variable_bindings/declare.html"><strong aria-hidden="true">4.3.</strong> Declare first</a></li></ol></li><li><a href="types.html"><strong aria-hidden="true">5.</strong> Types</a></li><li><ol class="section"><li><a href="types/cast.html"><strong aria-hidden="true">5.1.</strong> Casting</a></li><li><a href="types/literals.html"><strong aria-hidden="true">5.2.</strong> Literals</a></li><li><a href="types/inference.html"><strong aria-hidden="true">5.3.</strong> Inference</a></li><li><a href="types/alias.html"><strong aria-hidden="true">5.4.</strong> Aliasing</a></li></ol></li><li><a href="conversion.html"><strong aria-hidden="true">6.</strong> Conversion</a></li><li><ol class="section"><li><a href="conversion/from_into.html"><strong aria-hidden="true">6.1.</strong> From and Into</a></li><li><a href="conversion/string.html"><strong aria-hidden="true">6.2.</strong> To and from Strings</a></li></ol></li><li><a href="expression.html"><strong aria-hidden="true">7.</strong> Expressions</a></li><li><a href="flow_control.html"><strong aria-hidden="true">8.</strong> Flow Control</a></li><li><ol class="section"><li><a href="flow_control/if_else.html"><strong aria-hidden="true">8.1.</strong> if/else</a></li><li><a href="flow_control/loop.html"><strong aria-hidden="true">8.2.</strong> loop</a></li><li><ol class="section"><li><a href="flow_control/loop/nested.html"><strong aria-hidden="true">8.2.1.</strong> Nesting and labels</a></li><li><a href="flow_control/loop/return.html"><strong aria-hidden="true">8.2.2.</strong> Returning from loops</a></li></ol></li><li><a href="flow_control/while.html"><strong aria-hidden="true">8.3.</strong> while</a></li><li><a href="flow_control/for.html"><strong aria-hidden="true">8.4.</strong> for and range</a></li><li><a href="flow_control/match.html"><strong aria-hidden="true">8.5.</strong> match</a></li><li><ol class="section"><li><a href="flow_control/match/destructuring.html"><strong aria-hidden="true">8.5.1.</strong> Destructuring</a></li><li><ol class="section"><li><a href="flow_control/match/destructuring/destructure_tuple.html"><strong aria-hidden="true">8.5.1.1.</strong> tuples</a></li><li><a href="flow_control/match/destructuring/destructure_enum.html"><strong aria-hidden="true">8.5.1.2.</strong> enums</a></li><li><a href="flow_control/match/destructuring/destructure_pointers.html"><strong aria-hidden="true">8.5.1.3.</strong> pointers/ref</a></li><li><a href="flow_control/match/destructuring/destructure_structures.html"><strong aria-hidden="true">8.5.1.4.</strong> structs</a></li></ol></li><li><a href="flow_control/match/guard.html"><strong aria-hidden="true">8.5.2.</strong> Guards</a></li><li><a href="flow_control/match/binding.html"><strong aria-hidden="true">8.5.3.</strong> Binding</a></li></ol></li><li><a href="flow_control/if_let.html"><strong aria-hidden="true">8.6.</strong> if let</a></li><li><a href="flow_control/while_let.html"><strong aria-hidden="true">8.7.</strong> while let</a></li></ol></li><li><a href="fn.html"><strong aria-hidden="true">9.</strong> Functions</a></li><li><ol class="section"><li><a href="fn/methods.html"><strong aria-hidden="true">9.1.</strong> Methods</a></li><li><a href="fn/closures.html"><strong aria-hidden="true">9.2.</strong> Closures</a></li><li><ol class="section"><li><a href="fn/closures/capture.html"><strong aria-hidden="true">9.2.1.</strong> Capturing</a></li><li><a href="fn/closures/input_parameters.html"><strong aria-hidden="true">9.2.2.</strong> As input parameters</a></li><li><a href="fn/closures/anonymity.html"><strong aria-hidden="true">9.2.3.</strong> Type anonymity</a></li><li><a href="fn/closures/input_functions.html"><strong aria-hidden="true">9.2.4.</strong> Input functions</a></li><li><a href="fn/closures/output_parameters.html"><strong aria-hidden="true">9.2.5.</strong> As output parameters</a></li><li><a href="fn/closures/closure_examples.html"><strong aria-hidden="true">9.2.6.</strong> Examples in std</a></li><li><ol class="section"><li><a href="fn/closures/closure_examples/iter_any.html"><strong aria-hidden="true">9.2.6.1.</strong> Iterator::any</a></li><li><a href="fn/closures/closure_examples/iter_find.html"><strong aria-hidden="true">9.2.6.2.</strong> Iterator::find</a></li></ol></li></ol></li><li><a href="fn/hof.html"><strong aria-hidden="true">9.3.</strong> Higher Order Functions</a></li><li><a href="fn/diverging.html"><strong aria-hidden="true">9.4.</strong> Diverging functions</a></li></ol></li><li><a href="mod.html"><strong aria-hidden="true">10.</strong> Modules</a></li><li><ol class="section"><li><a href="mod/visibility.html"><strong aria-hidden="true">10.1.</strong> Visibility</a></li><li><a href="mod/struct_visibility.html"><strong aria-hidden="true">10.2.</strong> Struct visibility</a></li><li><a href="mod/use.html"><strong aria-hidden="true">10.3.</strong> The use declaration</a></li><li><a href="mod/super.html"><strong aria-hidden="true">10.4.</strong> super and self</a></li><li><a href="mod/split.html"><strong aria-hidden="true">10.5.</strong> File hierarchy</a></li></ol></li><li><a href="crates.html"><strong aria-hidden="true">11.</strong> Crates</a></li><li><ol class="section"><li><a href="crates/lib.html"><strong aria-hidden="true">11.1.</strong> Library</a></li><li><a href="crates/link.html"><strong aria-hidden="true">11.2.</strong> extern crate</a></li></ol></li><li><a href="cargo.html"><strong aria-hidden="true">12.</strong> Cargo</a></li><li><ol class="section"><li><a href="cargo/deps.html"><strong aria-hidden="true">12.1.</strong> Dependencies</a></li><li><a href="cargo/conventions.html"><strong aria-hidden="true">12.2.</strong> Conventions</a></li><li><a href="cargo/test.html"><strong aria-hidden="true">12.3.</strong> Tests</a></li></ol></li><li><a href="attribute.html"><strong aria-hidden="true">13.</strong> Attributes</a></li><li><ol class="section"><li><a href="attribute/unused.html"><strong aria-hidden="true">13.1.</strong> dead_code</a></li><li><a href="attribute/crate.html"><strong aria-hidden="true">13.2.</strong> Crates</a></li><li><a href="attribute/cfg.html"><strong aria-hidden="true">13.3.</strong> cfg</a></li><li><ol class="section"><li><a href="attribute/cfg/custom.html"><strong aria-hidden="true">13.3.1.</strong> Custom</a></li></ol></li></ol></li><li><a href="generics.html"><strong aria-hidden="true">14.</strong> Generics</a></li><li><ol class="section"><li><a href="generics/gen_fn.html"><strong aria-hidden="true">14.1.</strong> Functions</a></li><li><a href="generics/impl.html"><strong aria-hidden="true">14.2.</strong> Implementation</a></li><li><a href="generics/gen_trait.html"><strong aria-hidden="true">14.3.</strong> Traits</a></li><li><a href="generics/bounds.html"><strong aria-hidden="true">14.4.</strong> Bounds</a></li><li><ol class="section"><li><a href="generics/bounds/testcase_empty.html"><strong aria-hidden="true">14.4.1.</strong> Testcase: empty bounds</a></li></ol></li><li><a href="generics/multi_bounds.html"><strong aria-hidden="true">14.5.</strong> Multiple bounds</a></li><li><a href="generics/where.html"><strong aria-hidden="true">14.6.</strong> Where clauses</a></li><li><a href="generics/new_types.html"><strong aria-hidden="true">14.7.</strong> New Type Idiom</a></li><li><a href="generics/assoc_items.html"><strong aria-hidden="true">14.8.</strong> Associated items</a></li><li><ol class="section"><li><a href="generics/assoc_items/the_problem.html"><strong aria-hidden="true">14.8.1.</strong> The Problem</a></li><li><a href="generics/assoc_items/types.html"><strong aria-hidden="true">14.8.2.</strong> Associated types</a></li></ol></li><li><a href="generics/phantom.html"><strong aria-hidden="true">14.9.</strong> Phantom type parameters</a></li><li><ol class="section"><li><a href="generics/phantom/testcase_units.html"><strong aria-hidden="true">14.9.1.</strong> Testcase: unit clarification</a></li></ol></li></ol></li><li><a href="scope.html"><strong aria-hidden="true">15.</strong> Scoping rules</a></li><li><ol class="section"><li><a href="scope/raii.html"><strong aria-hidden="true">15.1.</strong> RAII</a></li><li><a href="scope/move.html"><strong aria-hidden="true">15.2.</strong> Ownership and moves</a></li><li><ol class="section"><li><a href="scope/move/mut.html"><strong aria-hidden="true">15.2.1.</strong> Mutability</a></li></ol></li><li><a href="scope/borrow.html"><strong aria-hidden="true">15.3.</strong> Borrowing</a></li><li><ol class="section"><li><a href="scope/borrow/mut.html"><strong aria-hidden="true">15.3.1.</strong> Mutability</a></li><li><a href="scope/borrow/freeze.html"><strong aria-hidden="true">15.3.2.</strong> Freezing</a></li><li><a href="scope/borrow/alias.html"><strong aria-hidden="true">15.3.3.</strong> Aliasing</a></li><li><a href="scope/borrow/ref.html"><strong aria-hidden="true">15.3.4.</strong> The ref pattern</a></li></ol></li><li><a href="scope/lifetime.html"><strong aria-hidden="true">15.4.</strong> Lifetimes</a></li><li><ol class="section"><li><a href="scope/lifetime/explicit.html"><strong aria-hidden="true">15.4.1.</strong> Explicit annotation</a></li><li><a href="scope/lifetime/fn.html"><strong aria-hidden="true">15.4.2.</strong> Functions</a></li><li><a href="scope/lifetime/methods.html"><strong aria-hidden="true">15.4.3.</strong> Methods</a></li><li><a href="scope/lifetime/struct.html"><strong aria-hidden="true">15.4.4.</strong> Structs</a></li><li><a href="scope/lifetime/lifetime_bounds.html"><strong aria-hidden="true">15.4.5.</strong> Bounds</a></li><li><a href="scope/lifetime/lifetime_coercion.html"><strong aria-hidden="true">15.4.6.</strong> Coercion</a></li><li><a href="scope/lifetime/static_lifetime.html"><strong aria-hidden="true">15.4.7.</strong> Static</a></li><li><a href="scope/lifetime/elision.html"><strong aria-hidden="true">15.4.8.</strong> Elision</a></li></ol></li></ol></li><li><a href="trait.html"><strong aria-hidden="true">16.</strong> Traits</a></li><li><ol class="section"><li><a href="trait/derive.html"><strong aria-hidden="true">16.1.</strong> Derive</a></li><li><a href="trait/ops.html"><strong aria-hidden="true">16.2.</strong> Operator Overloading</a></li><li><a href="trait/drop.html"><strong aria-hidden="true">16.3.</strong> Drop</a></li><li><a href="trait/iter.html"><strong aria-hidden="true">16.4.</strong> Iterators</a></li><li><a href="trait/clone.html"><strong aria-hidden="true">16.5.</strong> Clone</a></li></ol></li><li><a href="macros.html"><strong aria-hidden="true">17.</strong> macro_rules!</a></li><li><ol class="section"><li><a href="macros/syntax.html"><strong aria-hidden="true">17.1.</strong> Syntax</a></li><li><ol class="section"><li><a href="macros/designators.html"><strong aria-hidden="true">17.1.1.</strong> Designators</a></li><li><a href="macros/overload.html"><strong aria-hidden="true">17.1.2.</strong> Overload</a></li><li><a href="macros/repeat.html"><strong aria-hidden="true">17.1.3.</strong> Repeat</a></li></ol></li><li><a href="macros/dry.html"><strong aria-hidden="true">17.2.</strong> DRY (Don't Repeat Yourself)</a></li><li><a href="macros/dsl.html"><strong aria-hidden="true">17.3.</strong> DSL (Domain Specific Languages)</a></li><li><a href="macros/variadics.html"><strong aria-hidden="true">17.4.</strong> Variadics</a></li></ol></li><li><a href="error.html" class="active"><strong aria-hidden="true">18.</strong> Error handling</a></li><li><ol class="section"><li><a href="error/panic.html"><strong aria-hidden="true">18.1.</strong> panic</a></li><li><a href="error/option_unwrap.html"><strong aria-hidden="true">18.2.</strong> Option & unwrap</a></li><li><ol class="section"><li><a href="error/option_unwrap/map.html"><strong aria-hidden="true">18.2.1.</strong> Combinators: map</a></li><li><a href="error/option_unwrap/and_then.html"><strong aria-hidden="true">18.2.2.</strong> Combinators: and_then</a></li></ol></li><li><a href="error/result.html"><strong aria-hidden="true">18.3.</strong> Result</a></li><li><ol class="section"><li><a href="error/result/result_map.html"><strong aria-hidden="true">18.3.1.</strong> map for Result</a></li><li><a href="error/result/result_alias.html"><strong aria-hidden="true">18.3.2.</strong> aliases for Result</a></li><li><a href="error/result/early_returns.html"><strong aria-hidden="true">18.3.3.</strong> Early returns</a></li><li><a href="error/result/enter_question_mark.html"><strong aria-hidden="true">18.3.4.</strong> Introducing ?</a></li></ol></li><li><a href="error/multiple_error_types.html"><strong aria-hidden="true">18.4.</strong> Multiple error types</a></li><li><ol class="section"><li><a href="error/multiple_error_types/option_result.html"><strong aria-hidden="true">18.4.1.</strong> Pulling Results out of Options</a></li><li><a href="error/multiple_error_types/define_error_type.html"><strong aria-hidden="true">18.4.2.</strong> Defining an error type</a></li><li><a href="error/multiple_error_types/boxing_errors.html"><strong aria-hidden="true">18.4.3.</strong> Boxing errors</a></li><li><a href="error/multiple_error_types/reenter_question_mark.html"><strong aria-hidden="true">18.4.4.</strong> Other uses of ?</a></li><li><a href="error/multiple_error_types/wrap_error.html"><strong aria-hidden="true">18.4.5.</strong> Wrapping errors</a></li></ol></li><li><a href="error/iter_result.html"><strong aria-hidden="true">18.5.</strong> Iterating over Results</a></li></ol></li><li><a href="std.html"><strong aria-hidden="true">19.</strong> Std library types</a></li><li><ol class="section"><li><a href="std/box.html"><strong aria-hidden="true">19.1.</strong> Box, stack and heap</a></li><li><a href="std/vec.html"><strong aria-hidden="true">19.2.</strong> Vectors</a></li><li><a href="std/str.html"><strong aria-hidden="true">19.3.</strong> Strings</a></li><li><a href="std/option.html"><strong aria-hidden="true">19.4.</strong> Option</a></li><li><a href="std/result.html"><strong aria-hidden="true">19.5.</strong> Result</a></li><li><ol class="section"><li><a href="std/result/question_mark.html"><strong aria-hidden="true">19.5.1.</strong> ?</a></li></ol></li><li><a href="std/panic.html"><strong aria-hidden="true">19.6.</strong> panic!</a></li><li><a href="std/hash.html"><strong aria-hidden="true">19.7.</strong> HashMap</a></li><li><ol class="section"><li><a href="std/hash/alt_key_types.html"><strong aria-hidden="true">19.7.1.</strong> Alternate/custom key types</a></li><li><a href="std/hash/hashset.html"><strong aria-hidden="true">19.7.2.</strong> HashSet</a></li></ol></li></ol></li><li><a href="std_misc.html"><strong aria-hidden="true">20.</strong> Std misc</a></li><li><ol class="section"><li><a href="std_misc/threads.html"><strong aria-hidden="true">20.1.</strong> Threads</a></li><li><ol class="section"><li><a href="std_misc/threads/testcase_mapreduce.html"><strong aria-hidden="true">20.1.1.</strong> Testcase: map-reduce</a></li></ol></li><li><a href="std_misc/channels.html"><strong aria-hidden="true">20.2.</strong> Channels</a></li><li><a href="std_misc/path.html"><strong aria-hidden="true">20.3.</strong> Path</a></li><li><a href="std_misc/file.html"><strong aria-hidden="true">20.4.</strong> File I/O</a></li><li><ol class="section"><li><a href="std_misc/file/open.html"><strong aria-hidden="true">20.4.1.</strong> open</a></li><li><a href="std_misc/file/create.html"><strong aria-hidden="true">20.4.2.</strong> create</a></li></ol></li><li><a href="std_misc/process.html"><strong aria-hidden="true">20.5.</strong> Child processes</a></li><li><ol class="section"><li><a href="std_misc/process/pipe.html"><strong aria-hidden="true">20.5.1.</strong> Pipes</a></li><li><a href="std_misc/process/wait.html"><strong aria-hidden="true">20.5.2.</strong> Wait</a></li></ol></li><li><a href="std_misc/fs.html"><strong aria-hidden="true">20.6.</strong> Filesystem Operations</a></li><li><a href="std_misc/arg.html"><strong aria-hidden="true">20.7.</strong> Program arguments</a></li><li><ol class="section"><li><a href="std_misc/arg/matching.html"><strong aria-hidden="true">20.7.1.</strong> Argument parsing</a></li></ol></li><li><a href="std_misc/ffi.html"><strong aria-hidden="true">20.8.</strong> Foreign Function Interface</a></li></ol></li><li><a href="testing.html"><strong aria-hidden="true">21.</strong> Testing</a></li><li><ol class="section"><li><a href="testing/unit_testing.html"><strong aria-hidden="true">21.1.</strong> Unit testing</a></li><li><a href="testing/doc_testing.html"><strong aria-hidden="true">21.2.</strong> Documentation testing</a></li><li><a href="testing/integration_testing.html"><strong aria-hidden="true">21.3.</strong> Integration testing</a></li><li><a href="testing/dev_dependencies.html"><strong aria-hidden="true">21.4.</strong> Dev-dependencies</a></li></ol></li><li><a href="meta.html"><strong aria-hidden="true">22.</strong> Meta</a></li><li><ol class="section"><li><a href="meta/doc.html"><strong aria-hidden="true">22.1.</strong> Documentation</a></li></ol></li><li><a href="unsafe.html"><strong aria-hidden="true">23.</strong> Unsafe Operations</a></li></ol>
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<a class="header" href="error.html#error-handling" id="error-handling"><h1>Error handling</h1></a>
<p>Error handling is the process of handling the possibility of failure. For
example, failing to read a file and then continuing to use that <em>bad</em> input
would clearly be problematic. Noticing and explicitly managing those errors
saves the rest of the program from various pitfalls.</p>
<p>There are various ways to deal with errors in Rust, which are described in the
following subchapters. They all have more or less subtle differences and different
use cases. As a rule of thumb:</p>
<p>An explicit <code>panic</code> is mainly useful for tests and dealing with unrecoverable errors.
For prototyping it can be useful, for example when dealing with functions that
haven't been implemented yet, but in those cases the more descriptive <code>unimplemented</code>
is better. In tests <code>panic</code> is a reasonable way to explicitly fail.</p>
<p>The <code>Option</code> type is for when a value is optional or when the lack of a value is
not an error condition. For example the parent of a directory - <code>/</code> and <code>C:</code> don't
have one. When dealing with <code>Option</code>s, <code>unwrap</code> is fine for prototyping and cases
where it's absolutely certain that there is guaranteed to be a value. However <code>expect</code>
is more useful since it lets you specify an error message in case something goes
wrong anyway.</p>
<p>When there is a chance that things do go wrong and the caller has to deal with the
problem, use <code>Result</code>. You can <code>unwrap</code> and <code>expect</code> them as well (please don't
do that unless it's a test or quick prototype).</p>
<p>For a more rigorous discussion of error handling, refer to the error
handling section in the <a href="https://doc.rust-lang.org/book/second-edition/ch09-00-error-handling.html">official book</a>.</p>
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