CODEWITHSUNDEEP
genericsrustnom
swaltek
swaltek

Reputation: 153

Specifying only 1 type argument to generic?

In an attempt to write a parser in rust using nom. I ran into this error

error[E0283]: type annotations needed
   --> src/parser.rs:8:22
    |
8   |     let mut parser = delimited(tag("<"), take_until(">"), tag(">"));
    |         ----------   ^^^^^^^^^ cannot infer type for type parameter `E` declared on the function `delimited`
    |         |
    |         consider giving `parser` a type
    |
    = note: cannot satisfy `_: ParseError<&str>`
note: required by a bound in `delimited`
   --> /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/nom-7.1.1/src/sequence/mod.rs:172:36
    |
172 | pub fn delimited<I, O1, O2, O3, E: ParseError<I>, F, G, H>(
    |                                    ^^^^^^^^^^^^^ required by this bound in `delimited`
help: consider specifying the type arguments in the function call
    |
8   |     let mut parser = delimited::<I, O1, O2, O3, E, F, G, H>(tag("<"), take_until(">"), tag(">"));
    |

Fair enough, I got this to compile and return what I need it to by changing the line of code to

let mut parser = delimited::<_,_,_,_, VerboseError<_>,_,_,_>(tag("<"), take_until(">"), tag(">"));

However, this seems like a pretty ugly piece of code. Is there a better way of writing this? A way to specify only the type parameter E rather then listing out all the other parameters with an underscore?

Upvotes: 1

Views: 2051

Answers (1)

Chayim Friedman
Chayim Friedman

Reputation: 71605

You can define a function that wraps delimited() but with more specific generics. Other than that, there isn't much you can do.

Upvotes: 2

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