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Add an example to read an environment variable #422

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45 changes: 44 additions & 1 deletion src/basics.md
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Expand Up @@ -38,7 +38,8 @@
| [Parse string into DateTime struct][ex-parse-datetime] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
| [Perform checked date and time calculations][ex-datetime-arithmetic] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
| [Examine the date and time][ex-examine-date-and-time] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
| [File names that have been modified in the last 24 hours for the working directory][ex-file-24-hours-modified] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] [![cat-os-badge]][cat-os] |
| [File names that have been modified in the last 24 hours for the working directory][ex-file-24-hours-modified] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] [![cat-os-badge]][cat-os] |
| [Read environment variable][ex-read-environment-variable] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] |

[ex-std-read-lines]: #ex-std-read-lines
<a name="ex-std-read-lines"></a>
Expand Down Expand Up @@ -1770,6 +1771,48 @@ fn run() -> Result<()> {
# quick_main!(run);
```

[ex-read-environment-variable]: #ex-read-environment-variable
<a name="ex-read-environment-variable"></a>
## Read environment variable
[![std-badge]][std] [![cat-filesystem-badge]][cat-filesystem]

Set an environment variable and read it back. If the variable is not found, then fallback
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I'd suggest to link to std documentation for std::env::set_var and std::env::var

to a default value.

```rust
# #[macro_use]
# extern crate error_chain;
use std::env;
use std::path::PathBuf;

# error_chain! {
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this example can be completely rewritten without error_chain crate just change fn run() -> Result<()> to fn main() and remove Ok(()) and quick_main with all error chain specific code

# foreign_links {
# Utf8(std::str::Utf8Error);
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Neither of these are needed for this example

# AddrParse(std::net::AddrParseError);
# }
# }

fn set_env_variable() {
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I'm not really sold on using separate function to wrap one liner here.
Also the duplication of the "config.toml" string in two places might be slightly confusing how about we use different values?

env::set_var("CONFIG_FILE", "config.toml");
}

fn run() -> Result<()> {
set_env_variable();
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setting and reading the same env variable does not make a control flow that is close to being realistic. How about we set one variable and read another?


let default_config_file = "config.toml";

let config_file_path = match env::var("CONFIG_FILE") {
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This might a matter of taste but how about rewriting this simple transformation from match to Result combinators with something like:

    let config_file_path = env::var("CONFIG_FILE")
        .map(PathBuf::from)
        .unwrap_or_else(|_| default_config_file.into());

Ok(val) => PathBuf::from(val),
Err(_) => PathBuf::from(default_config_file)
};

println!("Env value is {}", config_file_path.display());
Ok(())
}
#
# quick_main!(run);
```

{{#include links.md}}

<!-- API Reference -->
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2 changes: 2 additions & 0 deletions src/intro.md
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Expand Up @@ -57,6 +57,7 @@ community. It needs and welcomes help. For details see
| [Perform checked date and time calculations][ex-datetime-arithmetic] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
| [Examine the date and time][ex-examine-date-and-time] | [![chrono-badge]][chrono] | [![cat-date-and-time-badge]][cat-date-and-time] |
| [File names that have been modified in the last 24 hours for the working directory][ex-file-24-hours-modified] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] [![cat-os-badge]][cat-os] |
| [Read environment variable][ex-read-environment-variable] | [![std-badge]][std] | [![cat-filesystem-badge]][cat-filesystem] |

## [Encoding](encoding.html)

Expand Down Expand Up @@ -193,6 +194,7 @@ community. It needs and welcomes help. For details see
[ex-extract-links-webpage]: net.html#ex-extract-links-webpage
[ex-extract-mediawiki-links]: net.html#ex-extract-mediawiki-links
[ex-file-24-hours-modified]: basics.html#ex-file-24-hours-modified
[ex-read-environment-variable]: basics.html#ex-read-environment-variable
[ex-file-post]: net.html#ex-file-post
[ex-file-predicate]: app.html#ex-file-predicate
[ex-file-sizes]: app.html#ex-file-sizes
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