Non-composability of GI.PangoCairo.fontMapGetDefault and GI.PangoCairo.fontMapGetResolution.

[deepfire]

According to https://hackage.haskell.org/package/gi-pangocairo-1.0.11/docs/src/GI-PangoCairo-Interfaces-FontMap.html#fontMapGetDefault :

fontMapGetDefault ::
    (B.CallStack.HasCallStack, MonadIO m) =>
    m Pango.FontMap.FontMap

..and..

fontMapGetResolution ::
    (B.CallStack.HasCallStack, MonadIO m, IsFontMap a) =>
    a
    {- ^ /@fontmap@/: a 'GI.PangoCairo.Interfaces.FontMap.FontMap' -}
    -> m Double
    {- ^ __Returns:__ the resolution in \"dots per inch\" -}

..which, in absence of some kind of hard-to-figure-out casting, makes these functions non-composable.

[deepfire]

Exhibit:

� GIPC.fontMapGetResolution =<< GIPC.fontMapGetDefault

<interactive>:36:1: error:
    • Required ancestor ‘GIPC.FontMap’ not found for type ‘GIP.FontMap’.
    • In the first argument of ‘(=<<)’, namely
        ‘GIPC.fontMapGetResolution’
      In the expression:
        GIPC.fontMapGetResolution =<< GIPC.fontMapGetDefault
      In an equation for ‘it’:
          it = GIPC.fontMapGetResolution =<< GIPC.fontMapGetDefault

[garetxe]

Thanks for the report.

As far as I can tell the issue here is with the design of the original API, the generated bindings are accurate. The problem is that there are two closely related types: Pango.FontMap and PangoCairo.FontMap. Every PangoCairo.FontMap is a Pango.FontMap, but not necessarily the other way round.

fontMapGetDefault returns a Pango.FontMap, and since this is not necessarily a PangoCairo.FontMap, the type system cannot ensure that the cast is safe, so you need to do it at runtime (admittedly the error message could hint that you may want to use a runtime cast, this may be a good thing to add). The following works, for example:

λ> import qualified GI.PangoCairo as PC
λ> import Data.GI.Base (unsafeCastTo)
λ> PC.fontMapGetDefault >>= unsafeCastTo PC.FontMap >>= PC.fontMapGetResolution
96.0

I am using unsafeCastTo here for conciseness, but if you can sensibly deal with the error case in your code, you may want to use castTo instead.

[deepfire]

I don't think this is the entire information -- quoting the documentation:

PangoFontMap * pango_cairo_font_map_get_default (void);

Gets a default PangoCairoFontMap to use with Cairo.

..so while the source code might only be returning a generic PangoFontMap, the documentation says it's a specific PangoCairoFontMap.

So, there goes. And, actually, I doubt it's the only case in the API, where the documentation is more precise than the C-level types.

And, given C's limitations, I guess we can all see the reason why the API providers are using the least common denominator..

[garetxe]

Looking to the relevant pangocairo source, it is clear that you are completely right. The API is intended to return a PangoCairo.FontMap. The issue is that the relevant methods (new, newFromFont and getDefault) are not marked as constructors in the introspection data. I added an override fixing this in 5492e21, and released gi-pangocairo-1.0.12 to hackage with the fix.

Thanks again for the report!

[deepfire]

Thank you a lot, once again!