In this case, I think the straightforward answer is that result is
clearly not a monad, just on the basis of its kind! However, for any
fixed x, fn a => result a x can be a monad. I suggest reformulating
your last definition to be polymorphic in that way. Probably everything
resolves most nicely if you swap the order of arguments to result.
On 2/6/20 1:34 PM, Urs Wegmann wrote:
I try to create a monad:
datatype result a x =
Ok of a
| Err of x
fun mreturn [a] [x] (r : a) : result a x = Ok r
fun mbind [a] [b] [x] (r : result a x) (f : a -> result b x) : result
b x =
case r of
Ok r => f r
| Err x => Err x
val result_monad = mkMonad
{
Return = @@mreturn,
Bind = @@mbind
}
I think the error message tries to tell me that x is not defined
enough. Is there a way to fix this? When I remove x from result, it
compiles.
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