Adding Type Annotations in patterns

[jrstrunk]

What does anyone think about adding the ability to add type annotations when pattern matching? An example of how this can be used straight from the radish package's decoder.gleam file is:

use #(length: BitArray, rest) <- result.then(consume_till_crlf(rest, <<>>))
use length: String <- result.then(bit_array.to_string(length))
use length: Int <- result.then(int.parse(length))

This is currently a compile error since I cannot tell the compiler that I am expecting the length variable from the consume_till_crlf function to be a BitArray. This might be nice because currently if in my code I am expecting a BitArray value for the first element, but the consume_till_crlf function returns something else, say a String, then I will get an error when I call bit_array.to_string, even though that may be correct in the context of my application and it is the consume_till_crlf function that is returning the wrong type.

In the current version of Gleam, the above code snippet actually looks like this:

use #(length, rest) <- result.then(consume_till_crlf(rest, <<>>))
use length <- result.then(bit_array.to_string(length))
use length <- result.then(int.parse(length))

*but could look like this:

use #(length, rest) <- result.then(consume_till_crlf(rest, <<>>))
use length: String <- result.then(bit_array.to_string(length))
use length: Int <- result.then(int.parse(length))

Which in my opinion is a little less easy to read because it is not obvious at a glance why length is redeclared three times.

So, this addition could add:

1. The ability to tell the compiler what we are expecting when pattern matching on variables, giving us more accurate error messages if something is wrong
2. Better readability for variables being redeclared after a type change

Any thoughts?

[lpil]

Hello!

*but could look like this:

use #(length, rest) <- result.then(consume_till_crlf(rest, <<>>))
use length: String <- result.then(bit_array.to_string(length))
use length: Int <- result.then(int.parse(length))

Which in my opinion is a little less easy to read because it is not obvious at a glance why length is redeclared three times.

This is valid code today.

The first example with the tuple annotation is written like so:

use #(length, rest): #(BitArray, _) <- result.then(consume_till_crlf(rest, <<>>))

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An aside: In Gleam it is called pattern matching rather than destructuring.

[jrstrunk]

Oh, that makes sense, thanks!

[lpil]

No prob!