value restriction
 Jacques Carette
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Date:  20061117 (03:12) 
From:  Jacques Carette <carette@m...> 
Subject:  value restriction 
I am wondering if there is a (nice) way around the value restriction in a particular case we have encountered. Inside a Functor, we compute a value as let make_result = match IF.R.fin with  Some rank_action > function m > perform rank < rank_action (); ret (Tuple.tup2 m.matrix rank)  None > failwith "Rank is not computed here" (where the above uses the campl4 extension for monadic notation). By design, we _want_ the failure to happen at Functor instantiation time (if it will fail), so the usual 'fix' of having make_result () does not help. But the code above does not work because of the value restriction :( Instead, we currently resort to let make_result m = perform rank < fromJust IF.R.fin (); ret (Tuple.tup2 m.matrix rank) (* Initialization: check the preconditions of instantiation of this struct*) let () = notNone IF.R.fin "Rank is not computed here" where let fromJust = function Some x > x  None > failwith "Can't happen" let notNone v str = match v with None > failwith str  Some _ > () which is clearly very ugly. So ugly that when 'Obj.magic' came up as a possibility around this, it was not immediately rejected. Yes, that ugly. The one possibility we have considered is to make IF.R.fin of type unit > 'a monad (no option) and instead of having None, we'd have fin () = failwith "Rank is not computed" Then we would have to do let make_result m = perform rank < IF.R.fin (); ret (Tuple.tup2 m.matrix rank) (* Initialization: check the preconditions of instantiation of this struct*) let _ = IF.R.fin () That would invoke IF.R.fin speculatively, and if that works, then invoke it 'for real' (but monadically the second time) later, when we know it will work. That certainly works, and has a certain appeal. And yet... Jacques