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Clarify what happens with numeric type inference

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Fixes #212.
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Carol (Nichols || Goulding) 2016-08-22 15:16:02 -04:00
parent c88a73688d
commit dbc3d3a140
1 changed files with 18 additions and 13 deletions
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src/ch02-00-guessing-game-tutorial.md
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@ -685,7 +685,9 @@ that `guess` should be a `String` and didn’t make us write the type out. Our
types which can have a value between one and a hundred: `i32`, a thirty-two-bit
number; or `u32`, an unsigned thirty-two-bit number; `i64`, a sixty-four-bit
number; or others. Rust defaults to an `i32`, so that's the type of
`secret_number`. The error is because Rust will not compare two different types.
`secret_number` unless we add type information elsewhere that would cause Rust
to infer a different numerical type. The error is because Rust will not compare
a string and a number type.
Ultimately, we want to convert the `String` we read as input
into a real number type so that we can compare it to the guess numerically. We
@ -757,21 +759,24 @@ number. Since this method can parse a variety of number types, we need to tell
Rust the exact type of number we want with `let guess: u32`. The colon (`:`)
after `guess` tells Rust we’re going to annotate its type. Rust has a few
built-in number types, but we’ve chosen `u32`, an unsigned, thirty-two bit
integer. It’s a good default choice for a small positive number. You'll see the
other number types in Chapter XX.
integer. It’s a good default choice for a small positive number. Additionally,
our `u32` annotation here and the comparison with `secret_number` means that
Rust will infer that `secret_number` should be a `u32` as well. So now the
comparison will be between two values of the same type!
[parse]: ../std/primitive.str.html#method.parse
Our call to `parse()` could quite easily cause an error, if, for example, our
string contained `A👍%`; there’d be no way to convert that to a number. Because
it might fail, the `parse()` method returns a `Result` type, much like the
`read_line()` method does that we discussed earlier. We're going to treat this
`Result` the same way by using the `expect()` method again. If `parse()`
returns an `Err` `Result` variant because it could not create a number from the
string, the `expect()` call will crash the game and print the message we give
it. If `parse()` can successfully turn the string into a number, it will return
the `Ok` variant of `Result`, and `expect()` will return the number that we
want that it will take out of the `Ok` value for us.
There's just one remaining piece to handle: our call to `parse()` could quite
easily cause an error, if, for example, our string contained `A👍%`; there’d be
no way to convert that to a number. Because it might fail, the `parse()` method
returns a `Result` type, much like the `read_line()` method does that we
discussed earlier. We're going to treat this `Result` the same way by using the
`expect()` method again. If `parse()` returns an `Err` `Result` variant because
it could not create a number from the string, the `expect()` call will crash
the game and print the message we give it. If `parse()` can successfully turn
the string into a number, it will return the `Ok` variant of `Result`, and
`expect()` will return the number that we want that it will take out of the
`Ok` value for us.
Let’s try our program out!