Add default value macro

README.md

@@ -118,12 +118,33 @@ your crate:
 extern crate dotenv_codegen;
 ```
 
-Then, in your crate:
+Then, in your crate you may retrieve a value like so:
 
 ```rust
 fn main() {
   println!("{}", dotenv!("MEANING_OF_LIFE"));
 }
 ```
 
+You may also panic with a supplied error message if the variable does
+not exist:
+
+```rust
+fn main() {
+    dotenv!("A_MISSING_VARIABLE", "This is an error message!");
+}
+```
+
+Or you can supply a default value if the variable does not exist:
+
+```rust
+fn main() {
+    let meaning_of_life: &str = dotenv_or_default!(
+        "A_MISSING_VARIABLE",
+        "42"
+    );
+    println!("{}", meaning_of_life);
+}
+```
+
 [dotenv]: https://github.com/bkeepers/dotenv

dotenv_codegen/src/lib.rs

@@ -1,4 +1,4 @@
 use proc_macro_hack::proc_macro_hack;
 
 #[proc_macro_hack]
-pub use dotenv_codegen_implementation::dotenv;
+pub use dotenv_codegen_implementation::{dotenv, dotenv_or_default};

dotenv_codegen/tests/basic_dotenv_macro.rs

@@ -17,3 +17,9 @@ fn two_argument_form_works() {
         "'quotes within quotes'"
     );
 }
+
+#[test]
+fn dotenv_or_default_works() {
+    let default_value: &str = dotenv_or_default!("CODEGEN_TEST_NONEXISTING_VARIABLE", "hello!");
+    assert_eq!(default_value, "hello!");
+}

dotenv_codegen_implementation/src/lib.rs

@@ -16,10 +16,38 @@ pub fn dotenv(input: TokenStream) -> TokenStream {
     }
 
     // Either everything was fine, or we didn't find an .env file (which we ignore)
-    expand_env(input)
+    let (var_name, second_value) = expand_env(input);
+
+    let err_msg = match second_value {
+        Some(e) => e,
+        None => format!("environment variable `{}` not defined", var_name),
+    };
+
+    match env::var(var_name) {
+        Ok(val) => quote!(#val).into(),
+        Err(VarError::NotPresent) | Err(VarError::NotUnicode(_)) => panic!("{}", err_msg),
+    }
 }
 
-fn expand_env(input_raw: TokenStream) -> TokenStream {
+#[proc_macro_hack]
+pub fn dotenv_or_default(input: TokenStream) -> TokenStream {
+    if let Err(err) = dotenv::dotenv() {
+        panic!("Error loading .env file: {}", err);
+    }
+
+    // Either everything was fine, or we didn't find an .env file (which we ignore)
+    let (var_name, second_value) = expand_env(input);
+    let default_val = match second_value {
+        Some(default) => default,
+        None => panic!("Missing default value for: {}", var_name),
+    };
+    match env::var(var_name) {
+        Ok(val) => quote!(#val).into(),
+        Err(VarError::NotPresent) | Err(VarError::NotUnicode(_)) => quote!(#default_val).into(),
+    }
+}
+
+fn expand_env(input_raw: TokenStream) -> (String, Option<String>) {
     let args = <Punctuated<syn::LitStr, Token![,]>>::parse_terminated
         .parse(input_raw)
         .expect("expected macro to be called with a comma-separated list of string literals");
@@ -31,17 +59,14 @@ fn expand_env(input_raw: TokenStream) -> TokenStream {
         None => panic!("expected 1 or 2 arguments, found none"),
     };
 
-    let err_msg = match iter.next() {
-        Some(lit) => lit.value(),
-        None => format!("environment variable `{}` not defined", var_name),
+    let second_value = match iter.next() {
+        Some(lit) => Some(lit.value()),
+        None => None,
     };
 
     if iter.next().is_some() {
         panic!("expected 1 or 2 arguments, found 3 or more");
     }
 
-    match env::var(var_name) {
-        Ok(val) => quote!(#val).into(),
-        Err(VarError::NotPresent) | Err(VarError::NotUnicode(_)) => panic!("{}", err_msg),
-    }
+    (var_name, second_value)
 }