python philosophical question - strong vs duck typing

Terry Reedy tjreedy at
Wed Jan 4 15:22:03 EST 2012

On 1/4/2012 1:37 AM, Terry Reedy wrote:
> On 1/3/2012 8:04 PM, Devin Jeanpierre wrote:

>> [ An example of a simple dependently typed program:
>> ]
> Just got it after a minute delay.

A followup now that I have read it. Removing the 40 line comment, the 
function itself is

fun getitem{n,m:nat}(arr : array(int, n) ,
  length : int(n), index : int m) : int =
     if index < length then
         ~1 (* -1, error *)

where n,m are compiler variables used to define the dependent 
(paramaterized) types array(int,n) and int(n)/ The double use of n means 
that the compiler checks that length n of the array equals the length 

My response: in Python, there is no need to pass concrete collection 
sizes because they are packaged with the collection at runtime as an 
attribute. So:

1) In Python, there is no need for such checking. In addition, the 
for-loop construct, 'for item in iterable:', removes the possibility of 
indexing errors.

2) Python classes are, in a sense, or in effect, runtime dependent 
types. While the formal implementation type of a 'list' is just 'list', 
the effective computation type is 'mutable sequence of length n'. The 
type of an iterator is 'read-only sequence of indefinite length'. I find 
this an interesting way to look at Python.

Terry Jan Reedy

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