[Ur] Troubles writing error monad

[Adam Chlipala]

On 06/09/2015 07:19 PM, Sergey Mironov wrote:
> Hi. I am trying to write an [Error] monad. So far I've managed to
> write the code (see below), which is rejected by the compiler

It's just a case of incompleteness in type inference.  An extra 
annotation sorts it all out.  In particular, this line of code

>    val monad_error = fn [e ::: Type] => (mkMonad { Return = @@mreturn [e], Bind = @@mbind [e] })

can be changed to this one:
   val monad_error = fn [e ::: Type] => (@@mkMonad [error e] { Return = 
@@mreturn [e], Bind = @@mbind [e] })