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From: Pierre Weis <Pierre.Weis@inria.fr>
Message-Id: <200011251554.QAA08379@pauillac.inria.fr>
Subject: Re: Typing the result of a function
In-Reply-To: <HDEEKOMJILGEIHIMAPCDKEMJDKAA.mattias.waldau@abc.se> from Mattias Waldau at "Nov 23, 100 11:25:32 pm"
To: mattias.waldau@abc.se (Mattias Waldau)
Date: Sat, 25 Nov 2000 16:54:13 +0100 (MET)
Cc: caml-list@inria.fr
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> I know how to type the arguments, and I like to do it, since I will get the
> compile errors directly, not first when I use the function.
> 
> Thus, I typical write a function like
> 
> (* return the column called name *)
> let find (columns:columns) (name:column_type) =
>   List.find ( fun column -> column.data_type = name ) columns.columns
> 
> which has typing
> 
> val find : columns -> column_type -> column = <fun>
> 
> If I use the interactive environment, I see that I get the expected result
> column. I needed to see this, since this is my first use of List.find, and I
> wanted to be sure that it returned the column.
> 
> However, I would have liked to say this already in the definition of find,
> that the result of my function find should be a column. How is this done?
>
> /mattias
> 
> P.s. I like to type, since I think it is belongs to the documentation of the
> code.

I would suggest to let those types in the module interfaces where
you should already have written the documentation of the code then ...

> P.P.s. How to type arguments is not very well described in the
> documentation. I found one example, twice.

Type constraints are simply introduced by a : symbol and should
normally be enclosed into parens (as in your code above). However, you
can use a simpler form to constraint the results of functions, just
write the constraints before the = sign:

let find (columns:columns) (name:column_type) : column =
   List.find ( fun column -> column.data_type = name ) columns.columns

Hope this helps,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/