I want to deserialize a form submission from a `ww...

# http4k

[Cies]

I want to deserialize a form submission from a www-form-urlencoded POST body received by a http4k server to a data class , preferably with kotlinx.serialization. I've done this before (when not using http4k) with Jackson, using quite some self written code, to (1) create a little language for adding structure to the form's name values (this allowed me to have nested data structures, and list/arrays, like .user.permissions[5]) and to parse the data directly to Jackson's internal JSON representation (in order to skip a JSON format step: Jackson was mainly used for it's "type coercion" since all values are received in one big www-form-urlencoded string).

Is there anything in http4k to make this easier? I've seen the body lens feature, but I have not been able to use it for form submissions.

[dave]

There isn't anything build in explicitly for this, but you could create something without too much effort I think. The problem you might have is that a JSON body effectively deserialises into a Map<String, Any> whereas the form body would deserialise into Map<String, String> . So - whatever you're using to deserialise needs to work around that conversion.

Assuming that you can work around that, you can use the httpBodyRoot() function to create a custom lens creation method. Here's the standard webform version (your one would be much simpler):

fun Body.Companion.webForm(
    validator: Validator,
    contentNegotiation: ContentNegotiation,
    vararg formFields: Lens<WebForm, *>
): BiDiBodyLensSpec<WebForm> =
    httpBodyRoot(formFields.map { it.meta }, APPLICATION_FORM_URLENCODED, contentNegotiation)
        .map({ it.payload.asString() }, { Body(it) })
        .map(
            { WebForm(formParametersFrom(it), emptyList()) },
            { (fields) -> fields.flatMap { pair -> pair.value.map { pair.key to it } }.toUrlFormEncoded() })
        .map(
            { it.copy(errors = validator(it, formFields.toList())) },
            { it.copy(errors = validator(it, formFields.toList())) })

For standard and multipart webforms, you use Body.webform().toLens() for the form and then <http://FormField.int|FormField.int>().required("whatever") etc.

[Cies]

Thanks, I'll have a look at that approach