Slight refactor of calculate_dice_amount. Lazy error handling.

Use unwrap_or_else instead of unwrap_or to prevent unnecessary error
string creation. Also some changes to make the code a bit more
readable.

src/dice.rs

@@ -6,26 +6,27 @@ use crate::parser::Amount;
 use crate::parser::Element as NewElement;
 use futures::stream::{self, StreamExt, TryStreamExt};
 
-//New hotness
+/// Calculate the amount of dice to roll by consulting the database
+/// and replacing variables with corresponding amounts. Errors out if
+/// it cannot find a variable defined, or if the database errors.
 pub async fn calculate_dice_amount(amounts: &[Amount], ctx: &Context<'_>) -> Result<i32, BotError> {
     let stream = stream::iter(amounts);
     let key = UserAndRoom(&ctx.username, ctx.room.id.as_str());
     let variables = &ctx.db.variables.get_user_variables(&key)?;
 
     use DiceRollingError::VariableNotFound;
-    let dice_amount: Result<i32, BotError> = stream
+    let dice_amount: i32 = stream
         .then(|amount| async move {
             match &amount.element {
-                NewElement::Number(num_dice) => Ok(*num_dice * amount.operator.mult()),
+                NewElement::Number(num_dice) => Ok(num_dice * amount.operator.mult()),
                 NewElement::Variable(variable) => variables
                     .get(variable)
-                    .ok_or(VariableNotFound(variable.clone().to_string()))
-                    .map(|i| *i)
-                    .map_err(|e| e.into()),
+                    .ok_or_else(|| VariableNotFound(variable.clone()))
+                    .map(|i| *i),
             }
         })
         .try_fold(0, |total, num_dice| async move { Ok(total + num_dice) })
-        .await;
+        .await?;
 
-    dice_amount
+    Ok(dice_amount)
 }