Please remember how types can be subtypes of other types :
consider
val f : int -> int
val g : int -> 'a
val h : 'a -> int
val i : 'a -> 'b
then the most general type is the type of g
and most specific type is type of h. In fact we have a diamond
relation between types :
h<f<g
h<i<g
and i and f are not related.
Why ?
because if you need a function of type like f (int -> int)
you can give h (if h can eat anything, it can eat int), but not g
(i don't now how to handle 'a if I only need a int).
So int is a subtype of 'a,
but int -> int is not a subtype of 'a -> 'b,
and 'a -> 'a is a valid subtype of int-> int.
Le 27 sept. 11 à 13:46, Walter Cazzola a écrit :
Dear all,
I'm still playing around with functors, modules and polymorphism but
I've some problems with this last concept.
In the attached files I have tried to implement a sort of function
with
a variable number of arguments (based on continuation) and to
generalize
the approach I've used a functor (OpVarADT) where I defined the
operation of type 'a -> 'b -> 'c but seems that it is not general
enough
to contain int->int->int or 'a -> 'a list -> 'a list
This is the functor instantiation with the errors I get:
# module M0 = Continuation(StringConcat) ;;
Error: Signature mismatch:
Modules do not match:
sig val op : 'a -> 'a list -> 'a list val init : 'a list end
is not included in
OpVarADT.OpVarADT
Values do not match:
val op : 'a -> 'a list -> 'a list
is not included in
val op : 'a -> 'b -> 'c
# module M1 = Continuation(Sum) ;;
Error: Signature mismatch:
Modules do not match:
sig val op : int -> int -> int val init : int end
is not included in
OpVarADT.OpVarADT
Values do not match:
val op : int -> int -> int
is not included in
val op : 'a -> 'b -> 'c
I'm sure I'm doing something wrong but I can't see what, any help is
welcome
TIA
Walter
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<sum.ml><OpVarADT.mli><continuation.ml><concat.ml>
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