I'm no longer entirely Haskell-incompetent, but in a current Yesod project which requires complex forms, I'm still liable to make a type error or two. Whereupon GHC gives this friendly message:
Handler/Activity.hs:90:0:
Couldn't match expected type `Maybe String'
against inferred type `FormType
(Control.Monad.Trans.Writer.Lazy.WriterT
Enctype
(Yesod.Form.Core.FormInner s m)
([Maybe String], FieldInfo s m))'
NB: `FormType' is a type function, and may not be injective
When generalising the type(s) for `modActivityForm'
Handler/Activity.hs:102:14:
Could not deduce (IsForm
(Control.Monad.Trans.Writer.Lazy.WriterT
Enctype
(Yesod.Form.Core.FormInner s m)
([Maybe String], FieldInfo s m)))
from the context ()
arising from a use of `maybeStringField'
at Handler/Activity.hs:102:14-56
Possible fix:
add (IsForm
(Control.Monad.Trans.Writer.Lazy.WriterT
Enctype
(Yesod.Form.Core.FormInner s m)
([Maybe String], FieldInfo s m))) to the context of
the type signature for `modActivityForm'
or add an instance declaration for
(IsForm
(Control.Monad.Trans.Writer.Lazy.WriterT
Enctype
(Yesod.Form.Core.FormInner s m)
([Maybe String], FieldInfo s m)))
In the first argument of `(.)', namely
`maybeStringField "Held resource to release"'
In the first argument of `mapM', namely
`(maybeStringField "Held resource to release" . Just)'
In the second argument of `($)', namely
`mapM
(maybeStringField "Held resource to release" . Just)
(take 5 $ marelNames ++ repeat Nothing)'
Handler/Activity.hs:90:0:
Couldn't match expected type `Maybe String'
against inferred type `FormType
(Control.Monad.Trans.Writer.Lazy.WriterT
Enctype
(Yesod.Form.Core.FormInner s m)
(FormResult ActivityResourceRelease, FieldInfo s m))'
NB: `FormType' is a type function, and may not be injective
When generalising the type(s) for `modActivityForm'
Often, the actual line of code with the nonsensical type signature isn't even listed as having an error.
After a day of poking around, I figured out that the error was in my own code, and simply appeared to be coming from the *Field functions. However, as is my habit when I suspect a type error in my code, I commented out most of my code, added dummy return types, and loaded the module into GHCi to inspect the return types... and got what looked like the same error!
I now understand that this message is a result of improper or insufficient constraints on the IsForm typeclass functions (or something along those lines), but discovering this on my own was painful. Any way this situation can result in less-intimidating error messages?
