#yesod

Yesod 1.6 was released in February of this year. This new version changes how subsites are implemented, which breaks most Yesod web applications. I did not find many good resources describing how to migrate web applications to 1.6, so I am documenting the steps I took to fix my website. It was not immediately obvious what changes needed to be made, so hopefully this is helpful for others.

My website was originally running on Stackage LTS 9.2, so I upgraded to LTS 12.2 by updating my stack.yaml file:

resolver: lts-12.2

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Yesod, a web framework for Haskell, provides convenient functionality that makes it simple to render and parse web forms. Unfortunately, it is not as elegant when it comes to implementing a POST handler that can process multiple web forms. This blog post presents one approach that simplifies the processing of multiple web forms in a single handler.

The first step to handling multiple POST forms is to use the identifyForm function. This function takes a string that uniquely identifies a given form and embeds a hidden field in the form (which is given as the second argument). When parsing a request’s POST data, if the hidden field’s identifier does not match the form’s unique identifier, parsing will return a FormMissing. This allows the request handler to parse other forms until one parses successfully. Here is an example of how to use identifyForm:

-- Data type for formA. 
data FormDataA = FormDataA Text

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