Add another another error exercise

README.md

@@ -99,6 +99,7 @@ The [Error Handling](https://doc.rust-lang.org/stable/book/error-handling.html)
 - ["result1.rs"](http://play.rust-lang.org/?code=%2F%2F+Make+this+test+pass%21+Scroll+down+for+hints+%3A%29%0A%0A%23%5Bderive%28PartialEq%2CDebug%29%5D%0Astruct+PositiveNonzeroInteger%28u64%29%3B%0A%0A%23%5Bderive%28PartialEq%2CDebug%29%5D%0Aenum+CreationError+%7B%0A++++Negative%2C%0A++++Zero%2C%0A%7D%0A%0Aimpl+PositiveNonzeroInteger+%7B%0A++++fn+new%28value%3A+i64%29+-%3E+Result%3CPositiveNonzeroInteger%2C+CreationError%3E+%7B%0A++++++++Ok%28PositiveNonzeroInteger%28value+as+u64%29%29%0A++++%7D%0A%7D%0A%0A%23%5Btest%5D%0Afn+test_creation%28%29+%7B%0A++++assert%21%28PositiveNonzeroInteger%3A%3Anew%2810%29.is_ok%28%29%29%3B%0A++++assert_eq%21%28Err%28CreationError%3A%3ANegative%29%2C+PositiveNonzeroInteger%3A%3Anew%28-10%29%29%3B%0A++++assert_eq%21%28Err%28CreationError%3A%3AZero%29%2C+PositiveNonzeroInteger%3A%3Anew%280%29%29%3B%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+%60PositiveNonzeroInteger%3A%3Anew%60+is+always+creating+a+new+instance+and+returning+an+%60Ok%60+result.%0A%2F%2F+It+should+be+doing+some+checking%2C+returning+an+%60Err%60+result+if+those+checks+fail%2C+and+only%0A%2F%2F+returning+an+%60Ok%60+result+if+those+checks+determine+that+everything+is...+okay+%3A%29%0A)
 - ["errors1.rs"](http://play.rust-lang.org/?code=%2F%2F+This+function+refuses+to+generate+text+to+be+printed+on+a+nametag+if%0A%2F%2F+you+pass+it+an+empty+string.+It%27d+be+nicer+if+it+explained+what+the+problem%0A%2F%2F+was%2C+instead+of+just+sometimes+returning+%60None%60.+The+2nd+test+currently%0A%2F%2F+does+not+compile+or+pass%2C+but+it+illustrates+the+behavior+we+would+like%0A%2F%2F+this+function+to+have.%0A%2F%2F+Scroll+down+for+hints%21%21%21%0A%0Apub+fn+generate_nametag_text%28name%3A+String%29+-%3E+Option%3CString%3E+%7B%0A++++if+name.len%28%29+%3E+0+%7B%0A++++++++Some%28format%21%28%22Hi%21+My+name+is+%7B%7D%22%2C+name%29%29%0A++++%7D+else+%7B%0A++++++++%2F%2F+Empty+names+aren%27t+allowed.%0A++++++++None%0A++++%7D%0A%7D%0A%0A%23%5Bcfg%28test%29%5D%0Amod+tests+%7B%0A++++use+super%3A%3A*%3B%0A%0A++++%2F%2F+This+test+passes+initially+if+you+comment+out+the+2nd+test.%0A++++%2F%2F+You%27ll+need+to+update+what+this+test+expects+when+you+change%0A++++%2F%2F+the+function+under+test%21%0A++++%23%5Btest%5D%0A++++fn+generates_nametag_text_for_a_nonempty_name%28%29+%7B%0A++++++++assert_eq%21%28%0A++++++++++++generate_nametag_text%28%22Beyonc%C3%A9%22.into%28%29%29%2C%0A++++++++++++Some%28%22Hi%21+My+name+is+Beyonc%C3%A9%22.into%28%29%29%0A++++++++%29%3B%0A++++%7D%0A%0A++++%23%5Btest%5D%0A++++fn+explains_why_generating_nametag_text_fails%28%29+%7B%0A++++++++assert_eq%21%28%0A++++++++++++generate_nametag_text%28%22%22.into%28%29%29%2C%0A++++++++++++Err%28%22%60name%60+was+empty%3B+it+must+be+nonempty.%22.into%28%29%29%0A++++++++%29%3B%0A++++%7D%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+%60Err%60+is+one+of+the+variants+of+%60Result%60%2C+so+what+the+2nd+test+is+saying%0A%2F%2F+is+that+%60generate_nametag_text%60+should+return+a+%60Result%60+instead+of+an%0A%2F%2F+%60Option%60.%0A%0A%2F%2F+To+make+this+change%2C+you%27ll+need+to%3A%0A%2F%2F+-+update+the+return+type+in+the+function+signature+to+be+a+Result+that%0A%2F%2F+++could+be+the+variants+%60Ok%28String%29%60+and+%60Err%28String%29%60%0A%2F%2F+-+change+the+body+of+the+function+to+return+%60Ok%28stuff%29%60+where+it+currently%0A%2F%2F+++returns+%60Some%28stuff%29%60%0A%2F%2F+-+change+the+body+of+the+function+to+return+%60Err%28error+message%29%60+where+it%0A%2F%2F+++currently+returns+%60None%60%0A%2F%2F+-+change+the+first+test+to+expect+%60Ok%28stuff%29%60+where+it+currently+expects%0A%2F%2F+++%60Some%28stuff%29%60.%0A)
 - ["errors2.rs"](http://play.rust-lang.org/?code=%2F%2F+Say+we%27re+writing+a+game+where+you+can+buy+items+with+tokens.+All+items+cost%0A%2F%2F+5+tokens%2C+and+whenever+you+purchase+items+there+is+a+processing+fee+of+1%0A%2F%2F+token.+A+player+of+the+game+will+type+in+how+many+items+they+want+to+buy%2C%0A%2F%2F+and+the+%60total_cost%60+function+will+calculate+the+total+number+of+tokens.%0A%2F%2F+Since+the+player+typed+in+the+quantity%2C+though%2C+we+get+it+as+a+string--+and%0A%2F%2F+they+might+have+typed+anything%2C+not+just+numbers%21%0A%0A%2F%2F+Right+now%2C+this+function+isn%27t+handling+the+error+case+at+all+%28and+isn%27t%0A%2F%2F+handling+the+success+case+properly+either%29.+What+we+want+to+do+is%3A%0A%2F%2F+if+we+call+the+%60parse%60+function+on+a+string+that+is+not+a+number%2C+that%0A%2F%2F+function+will+return+a+%60ParseIntError%60%2C+and+in+that+case%2C+we+want+to%0A%2F%2F+immediately+return+that+error+from+our+function+and+not+try+to+multiply%0A%2F%2F+and+add.%0A%0A%2F%2F+There+are+at+least+two+ways+to+implement+this+that+are+both+correct--+but%0A%2F%2F+one+is+a+lot+shorter%21+Scroll+down+for+hints+to+both+ways.%0A%0Ause+std%3A%3Anum%3A%3AParseIntError%3B%0A%0Apub+fn+total_cost%28item_quantity%3A+%26str%29+-%3E+Result%3Ci32%2C+ParseIntError%3E+%7B%0A++++let+processing_fee+%3D+1%3B%0A++++let+cost_per_item+%3D+5%3B%0A++++let+qty+%3D+item_quantity.parse%3A%3A%3Ci32%3E%28%29%3B%0A%0A++++Ok%28qty+*+cost_per_item+%2B+processing_fee%29%0A%7D%0A%0A%23%5Bcfg%28test%29%5D%0Amod+tests+%7B%0A++++use+super%3A%3A*%3B%0A%0A++++%23%5Btest%5D%0A++++fn+item_quantity_is_a_valid_number%28%29+%7B%0A++++++++assert_eq%21%28%0A++++++++++++total_cost%28%2234%22%29%2C%0A++++++++++++Ok%28171%29%0A++++++++%29%3B%0A++++%7D%0A%0A++++%23%5Btest%5D%0A++++fn+item_quantity_is_an_invalid_number%28%29+%7B%0A++++++++assert_eq%21%28%0A++++++++++++total_cost%28%22beep+boop%22%29.unwrap_err%28%29.to_string%28%29%2C%0A++++++++++++%22invalid+digit+found+in+string%22%0A++++++++%29%3B%0A++++%7D%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+One+way+to+handle+this+is+using+a+%60match%60+statement+on%0A%2F%2F+%60item_quantity.parse%3A%3A%3Ci32%3E%28%29%60+where+the+cases+are+%60Ok%28something%29%60+and%0A%2F%2F+%60Err%28something%29%60.+This+pattern+is+very+common+in+Rust%2C+though%2C+so+there%27s%0A%2F%2F+a+%60try%21%60+macro+that+does+pretty+much+what+you+would+make+that+match+statement%0A%2F%2F+do+for+you%21+Take+a+look+at+this+section+of+the+Error+Handling+chapter%3A%0A%2F%2F+https%3A%2F%2Fdoc.rust-lang.org%2Fstable%2Fbook%2Ferror-handling.html%23the-try-macro%0A%2F%2F+and+give+it+a+%60try%21%60%0A)
+- - ["errors3.rs"](http://play.rust-lang.org/?code=%2F%2F+This+is+a+program+that+is+trying+to+use+a+completed+version+of+the%0A%2F%2F+%60total_cost%60+function+from+the+previous+exercise.+It%27s+not+working+though--%0A%2F%2F+we+can%27t+call+the+%60try%21%60+macro+in+the+%60main%28%29%60+function%21+Why+not%3F%0A%2F%2F+What+should+we+do+instead%3F+Scroll+for+hints%21%0A%0Ause+std%3A%3Anum%3A%3AParseIntError%3B%0A%0Afn+main%28%29+%7B%0A++++let+mut+tokens+%3D+100%3B%0A++++let+pretend_user_input+%3D+%228%22%3B%0A%0A++++let+cost+%3D+try%21%28total_cost%28pretend_user_input%29%29%3B%0A%0A++++if+cost+%3E+tokens+%7B%0A++++++++println%21%28%22You+can%27t+afford+that+many%21%22%29%3B%0A++++%7D+else+%7B%0A++++++++tokens+-%3D+cost%3B%0A++++++++println%21%28%22You+now+have+%7B%7D+tokens.%22%2C+tokens%29%3B%0A++++%7D%0A%7D%0A%0Apub+fn+total_cost%28item_quantity%3A+%26str%29+-%3E+Result%3Ci32%2C+ParseIntError%3E+%7B%0A++++let+processing_fee+%3D+1%3B%0A++++let+cost_per_item+%3D+5%3B%0A++++let+qty+%3D+try%21%28item_quantity.parse%3A%3A%3Ci32%3E%28%29%29%3B%0A%0A++++Ok%28qty+*+cost_per_item+%2B+processing_fee%29%0A%7D%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%0A%2F%2F+Since+the+%60try%21%60+macro+returns+an+%60Err%60+early+if+the+thing+it%27s+trying+to%0A%2F%2F+do+fails%2C+you+can+only+use+the+%60try%21%60+macro+in+functions+that+have+a%0A%2F%2F+%60Result%60+as+their+return+type.%0A%0A%2F%2F+The+error+that+you+get+if+you+run+this+code+is%3A%0A%0A%2F%2F+%60%60%60%0A%2F%2F+error%3A+mismatched+types%3A%0A%2F%2F+expected+%60%28%29%60%2C%0A%2F%2F+found+%60std%3A%3Aresult%3A%3AResult%3C_%2C+_%3E%60%0A%2F%2F+%60%60%60%0A%0A%2F%2F+which+is+saying+that+the+expected+return+type+of+the+%60main%60+function+is%0A%2F%2F+the+empty+tuple%2C+but+we+tried+to+return+a+%60Result%60--+and+that%27s+happening%0A%2F%2F+in+the+implementation+of+%60try%21%60.+The+%60main%60+function+never+has+a+return+type%2C%0A%2F%2F+so+we+have+to+use+another+way+of+handling+a+%60Result%60+within+%60main%60.%0A%0A%2F%2F+Decide+what+we+should+do+if+%60pretend_user_input%60+has+a+string+value+that+does%0A%2F%2F+not+parse+to+an+integer%2C+and+implement+that+instead+of+calling+the+%60try%21%60%0A%2F%2F+macro.%0A)
 - ["errorsn.rs"](http://play.rust-lang.org/?code=%2F%2F+This+is+a+bigger+error+exercise+than+the+previous+ones%21%0A%2F%2F%0A%2F%2F+Edit+the+%60read_and_validate%60+function+so+that+it+compiles+and%0A%2F%2F+passes+the+tests...+so+many+things+could+go+wrong%21%0A%2F%2F%0A%2F%2F+-+Reading+from+stdin+could+produce+an+io%3A%3AError%0A%2F%2F+-+Parsing+the+input+could+produce+a+num%3A%3AParseIntError%0A%2F%2F+-+Validating+the+input+could+produce+a+CreationError+%28defined+below%29%0A%2F%2F%0A%2F%2F+How+can+we+lump+these+errors+into+one+general+error%3F+That+is%2C+what%0A%2F%2F+type+goes+where+the+question+marks+are%2C+and+how+do+we+return%0A%2F%2F+that+type+from+the+body+of+read_and_validate%3F%0A%2F%2F%0A%2F%2F+Scroll+down+for+hints+%3A%29%0A%0Ause+std%3A%3Aerror%3B%0Ause+std%3A%3Afmt%3B%0Ause+std%3A%3Aio%3B%0A%0A%2F%2F+PositiveNonzeroInteger+is+a+struct+defined+below+the+tests.%0Afn+read_and_validate%28b%3A+%26mut+io%3A%3ABufRead%29+-%3E+Result%3CPositiveNonzeroInteger%2C+%3F%3F%3F%3E+%7B%0A++++let+mut+line+%3D+String%3A%3Anew%28%29%3B%0A++++b.read_line%28%26mut+line%29%3B%0A++++let+num%3A+i64+%3D+line.trim%28%29.parse%28%29%3B%0A++++PositiveNonzeroInteger%3A%3Anew%28num%29%0A%7D%0A%0A%2F%2F+This+is+a+test+helper+function+that+turns+a+%26str+into+a+BufReader.%0Afn+test_with_str%28s%3A+%26str%29+-%3E+Result%3CPositiveNonzeroInteger%2C+Box%3Cerror%3A%3AError%3E%3E+%7B%0A++++let+mut+b+%3D+io%3A%3ABufReader%3A%3Anew%28s.as_bytes%28%29%29%3B%0A++++read_and_validate%28%26mut+b%29%0A%7D%0A%0A%23%5Btest%5D%0Afn+test_success%28%29+%7B%0A++++let+x+%3D+test_with_str%28%2242%5Cn%22%29%3B%0A++++assert_eq%21%28PositiveNonzeroInteger%2842%29%2C+x.unwrap%28%29%29%3B%0A%7D%0A%0A%23%5Btest%5D%0Afn+test_not_num%28%29+%7B%0A++++let+x+%3D+test_with_str%28%22eleven+billion%5Cn%22%29%3B%0A++++assert%21%28x.is_err%28%29%29%3B%0A%7D%0A%0A%23%5Btest%5D%0Afn+test_non_positive%28%29+%7B%0A++++let+x+%3D+test_with_str%28%22-40%5Cn%22%29%3B%0A++++assert%21%28x.is_err%28%29%29%3B%0A%7D%0A%0A%23%5Btest%5D%0Afn+test_ioerror%28%29+%7B%0A++++struct+Broken%3B%0A++++impl+io%3A%3ARead+for+Broken+%7B%0A++++++++fn+read%28%26mut+self%2C+_buf%3A+%26mut+%5Bu8%5D%29+-%3E+io%3A%3AResult%3Cusize%3E+%7B%0A++++++++++++Err%28io%3A%3AError%3A%3Anew%28io%3A%3AErrorKind%3A%3ABrokenPipe%2C+%22uh-oh%21%22%29%29%0A++++++++%7D%0A++++%7D%0A++++let+mut+b+%3D+io%3A%3ABufReader%3A%3Anew%28Broken%29%3B%0A++++assert%21%28read_and_validate%28%26mut+b%29.is_err%28%29%29%3B%0A%7D%0A%0A%23%5Bderive%28PartialEq%2CDebug%29%5D%0Astruct+PositiveNonzeroInteger%28u64%29%3B%0A%0Aimpl+PositiveNonzeroInteger+%7B%0A++++fn+new%28value%3A+i64%29+-%3E+Result%3CPositiveNonzeroInteger%2C+CreationError%3E+%7B%0A++++++++if+value+%3D%3D+0+%7B%0A++++++++++++Err%28CreationError%3A%3AZero%29%0A++++++++%7D+else+if+value+%3C+0+%7B%0A++++++++++++Err%28CreationError%3A%3ANegative%29%0A++++++++%7D+else+%7B%0A++++++++++++Ok%28PositiveNonzeroInteger%28value+as+u64%29%29%0A++++++++%7D%0A++++%7D%0A%7D%0A%0A%23%5Btest%5D%0Afn+test_positive_nonzero_integer_creation%28%29+%7B%0A++++assert%21%28PositiveNonzeroInteger%3A%3Anew%2810%29.is_ok%28%29%29%3B%0A++++assert_eq%21%28Err%28CreationError%3A%3ANegative%29%2C+PositiveNonzeroInteger%3A%3Anew%28-10%29%29%3B%0A++++assert_eq%21%28Err%28CreationError%3A%3AZero%29%2C+PositiveNonzeroInteger%3A%3Anew%280%29%29%3B%0A%7D%0A%0A%23%5Bderive%28PartialEq%2CDebug%29%5D%0Aenum+CreationError+%7B%0A++++Negative%2C%0A++++Zero%2C%0A%7D%0A%0Aimpl+fmt%3A%3ADisplay+for+CreationError+%7B%0A++++fn+fmt%28%26self%2C+f%3A+%26mut+fmt%3A%3AFormatter%29+-%3E+fmt%3A%3AResult+%7B%0A++++++++f.write_str%28%28self+as+%26error%3A%3AError%29.description%28%29%29%0A++++%7D%0A%7D%0A%0Aimpl+error%3A%3AError+for+CreationError+%7B%0A++++fn+description%28%26self%29+-%3E+%26str+%7B%0A++++++++match+*self+%7B%0A++++++++++++CreationError%3A%3ANegative+%3D%3E+%22Negative%22%2C%0A++++++++++++CreationError%3A%3AZero+%3D%3E+%22Zero%22%2C%0A++++++++%7D%0A++++%7D%0A%7D%0A%0A%2F%2F+First+hint%3A+To+figure+out+what+type+should+go+where+the+%3F%3F%3F+is%2C+take+a+look%0A%2F%2F+at+the+test+helper+function+%60test_with_str%60%2C+since+it+returns+whatever%0A%2F%2F+%60read_and_validate%60+returns+and%60test_with_str%60+has+its+signature+fully%0A%2F%2F+specified.%0A%0A%2F%2F+Next+hint%3A+anywhere+in+%60read_and_validate%60+that+we+call+a+function+that%0A%2F%2F+returns+a+%60Result%60%2C+wrap+that+call+in+a+%60try%21%60+macro+call.+Use+the+compiler%0A%2F%2F+error+messages+and+warnings+to+guide+you+to+all+the+places+you+need+to+do%0A%2F%2F+this.+You+might+need+to+rewrap+some+%60try%21%60+return+values+in+a+%60Result%3A%3AOk%60%21%0A%0A%2F%2F+This+works+because+under+the+hood%2C+the+%60try%21%60+macro+calls+%60From%3A%3Afrom%60%0A%2F%2F+on+the+error+value+to+convert+it+to+a+boxed+trait+object%2C+a+Box%3Cerror%3A%3AError%3E%2C%0A%2F%2F+which+is+polymorphic.%0A)
 
 ### Standard library types

error_handling/errors3.rs

@@ -0,0 +1,66 @@
+// This is a program that is trying to use a completed version of the
+// `total_cost` function from the previous exercise. It's not working though--
+// we can't call the `try!` macro in the `main()` function! Why not?
+// What should we do instead? Scroll for hints!
+
+use std::num::ParseIntError;
+
+fn main() {
+    let mut tokens = 100;
+    let pretend_user_input = "8";
+
+    let cost = try!(total_cost(pretend_user_input));
+
+    if cost > tokens {
+        println!("You can't afford that many!");
+    } else {
+        tokens -= cost;
+        println!("You now have {} tokens.", tokens);
+    }
+}
+
+pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
+    let processing_fee = 1;
+    let cost_per_item = 5;
+    let qty = try!(item_quantity.parse::<i32>());
+
+    Ok(qty * cost_per_item + processing_fee)
+}
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
+
+
+
+// Since the `try!` macro returns an `Err` early if the thing it's trying to
+// do fails, you can only use the `try!` macro in functions that have a
+// `Result` as their return type.
+
+// The error that you get if you run this code is:
+
+// ```
+// error: mismatched types:
+// expected `()`,
+// found `std::result::Result<_, _>`
+// ```
+
+// which is saying that the expected return type of the `main` function is
+// the empty tuple, but we tried to return a `Result`-- and that's happening
+// in the implementation of `try!`. The `main` function never has a return type,
+// so we have to use another way of handling a `Result` within `main`.
+
+// Decide what we should do if `pretend_user_input` has a string value that does
+// not parse to an integer, and implement that instead of calling the `try!`
+// macro.