Variant type with 'a type?

Variant type with 'a type?

[Robert Fischer]

Is it possible to do a variant type along the lines of "type try = Result of 'a | Exception of exn"?
   When I try that, I get an error saying "Unbound type parameter 'a".

~~ Robert.

Re: [Caml-list] Variant type with 'a type?

[Robert Fischer]

Figured it out -- "type 'a maybe = Result of 'a | Exception of exn"

Putting the tick-a before the "maybe" was nonintuitive to me.

~~ Robert.

Robert Fischer wrote:
> Is it possible to do a variant type along the lines of "type try = Result of 'a | Exception of exn"?
>    When I try that, I get an error saying "Unbound type parameter 'a".
> 
> ~~ Robert.
> 
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Re: [Caml-list] Variant type with 'a type?

[Brian Hurt]

On Mon, 9 Jun 2008, Robert Fischer wrote:

> Is it possible to do a variant type along the lines of "type try = Result of 'a | Exception of exn"?
>   When I try that, I get an error saying "Unbound type parameter 'a".

1) try is a key
2) paramaterize the type.

so you can do:
type 'a try_t =
 	| Result of 'a
 	| Exception of exn

And while you're digging at this spot, let me point out that you might 
want to define the module like this:

module Try = struct
 	let bind m f =
 		match m with
 		| Exception e -> Exception e
 		| Result x ->
 			try
 				f x
 			with
 			| e -> Exception e
 	;;

 	let ( >>= ) m f = bind m f;;

 	let return x = Result x;;

 	let fail s = Exception (Failure s)
end;;

Welcome to the wonderful world of monads.

Brian