Context

The IHP Guide has a section that describes their use of hash symbols:

https://ihp.digitallyinduced.com/Guide/helpful-tips.html

It mentions the following:

Writing #companyId is equivalent to writing fromLabel @"companyId".

Experiment

In the example blog application, if I add the following to a view:

{get #title post}

The title of the post shows up as expected.

Now, if I add the following to the view instead:

{get (fromLabel @"title") post}

the page doesn't render properly and it gives me the following message:

Web/View/Posts/Show.hs:11:33
    • 12:40:
   |
12 |         {get (fromLabel @"title") post}
   |                                        ^
"0\nSrcLoc \"\" 1 30\nParse error in expression: fromLabel@\"title\"\n"

Question

Based on the what is mentioned in the documentation cited above, it seemed that I could write:

get (fromLabel @"title") post

instead of:

get #title post

But that clearly does not work. :-)

So, what am I missing here? Is there a way to write:

get #title post

in terms of fromLabel?

Update 1

As suggested by Fyodor in a comment below, I tried the following:

{show @Int 42}

This was the result:

Web/View/Posts/Show.hs:11:33
    • 12:23:
   |
12 |         {show @Int 42}
   |                       ^
"0\nSrcLoc \"\" 1 13\nParse error in expression: show@Int\n"

CodePudding user response：

This is a known bug :-)

See https://github.com/digitallyinduced/ihp/issues/1049 and https://github.com/digitallyinduced/ihp/issues/857

Try writing it like this:

[hsx|{title}|]
    where
        title = get (fromLabel @"title") post

Inside HSX expressions the haskell code inside {} is parsed using a custom haskell parser (not the one used by the haskell compiler). The custom parser is a bit outdated and doesn't support the @SomeType syntax yet.

The #hash syntax is also not supported directly by the parser. The parser thinks that the # symbol is an infix operator (like or -). HSX later transforms the infix expressions with # into the correct fromLabel call.

With recent versions of the haskell compiler we can actually get rid of the third-party parser entirely and use the haskell compiler parser directly. We plan to do that in the future to fix this issue.