[Camllist] Weird types

Berke Durak
 TyngRuey Chuang
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Date:  20010616 (17:31) 
From:  TyngRuey Chuang <trc@i...> 
Subject:  Re: [Camllist] Weird types 
Berke Durak wrote: > > I have a type > > type ('a,'b,'c,'d,'e,'f,'g) sigma = > I of 'a * 'b * 'c * 'd * 'e * 'f * 'g >  T of ('b,'a,'c,'d,'e,'f,'g) sigma >  P of ('b,'c,'d,'e,'f,'g,'a) sigma > > and a function > > let rec gamma = function > I _ > 0  > T x > 1 + (gamma x)  > P x > 1 + (gamma x) > > and want a version of gamma that works on the following data > > type t1 = X1 and t2 = X2 and t3 = X3 and > t4 = X4 and t5 = X5 and t6 = X6 and t7 = X7 > > let data = I(X1,X2,X3,X4,X5,X6,X7) > > and that is under 10K of length. Any clever way to solve this ? .... I am not sure people at INRIA will recommend this, but one can use Obj.magic to coerce the compiler to accept unsafe value definitions. For example, let rec gamma s = match s with I _ > 0  T x > 1 + Obj.magic gamma x  P x > 1 + Obj.magic gamma x will be inferred as val gamma : ('a, 'b, 'c, 'd, 'e, 'f, 'g) sigma > int = <fun> In this particular case, function gamma is safe to have the above type because, by its definition, values of types 'a, 'b, 'c, 'd, 'e, 'f, and 'g are always ignored. If we define let i = I (X1, X2, X3, X4, X5, X6, X7) let rec t n = if n <= 0 then i else T (t' (n1)) and t' n = if n <= 0 then T i else T (t (n1)) then (t (2*k)) will return a "length (2*k)" sigma value, and (t' (2*k+1)) will return a "length (2*k+1)" sigma value. Functions t and t' are nicely inferred by the compiler to have types val t : int > (t1, t2, t3, t4, t5, t6, t7) sigma = <fun> val t' : int > (t2, t1, t3, t4, t5, t6, t7) sigma = <fun> Troubles are, (t (2*k1)) also has length (2*k). Also, (t' (2*k)) has length (2*k+1). This is no good, but one probably cannot do better. It can also be inferred that sigma values of the same length may not have the same type. (P (t (2*k))) and (t' (2*k+1)) both have length (2*k+1), but with different types: let t'10k1 = t' 10001 let t10k = t 10000 let pt10k = P t10k let (u, v) = (gamma pt10k, gamma t'10k1) We get val t'10k1 : (t2, t1, t3, t4, t5, t6, t7) sigma = ... val t10k : (t1, t2, t3, t4, t5, t6, t7) sigma = ... val pt10k : (t7, t1, t2, t3, t4, t5, t6) sigma = ... val u : int = 10001 val v : int = 10001 By the way, people call type constructors like sigma "irregular": sigma is applied to different type expresions at the two sides of the its own type equation. A selfcontained code fragment is appended for your amusement. Have fun! TyngRuey Chuang  type ('a, 'b, 'c, 'd, 'e, 'f, 'g) sigma = I of 'a * 'b * 'c * 'd *'e * 'f * 'g  T of ('b, 'a, 'c, 'd, 'e, 'f, 'g) sigma  P of ('b, 'c, 'd, 'e, 'f, 'g, 'a) sigma let rec gamma s = match s with I _ > 0  T x > 1 + Obj.magic gamma x  P x > 1 + Obj.magic gamma x type t1 = X1 and t2 = X2 and t3 = X3 and t4 = X4 and t5 = X5 and t6 = X6 and t7 = X7 let i = I (X1, X2, X3, X4, X5, X6, X7) let rec t n = if n <= 0 then i else T (t' (n1)) and t' n = if n <= 0 then T i else T (t (n1)) let t'10k1 = t' 10001 let t10k = t 10000 let pt10k = P t10k let (u, v) = (gamma pt10k, gamma t'10k1)  Bug reports: http://caml.inria.fr/bin/camlbugs FAQ: http://caml.inria.fr/FAQ/ To unsubscribe, mail camllistrequest@inria.fr Archives: http://caml.inria.fr