On Mon, Aug 14, 2017 at 9:40 AM, Chris Angelico <ros...@gmail.com> wrote:
> On Tue, Aug 15, 2017 at 1:33 AM, Ian Kelly <ian.g.ke...@gmail.com> wrote:
>> On Sun, Aug 13, 2017 at 8:36 AM, Steve D'Aprano
>>> Sure. In Haskell, comprehensions are *implicit* loops, rather than explicit
>>> like
>>> in Python.
>>
>> No, they aren't. Haskell list comprehensions use lazy evaluation.
>> Here's an example of an infinite list comprehension:
>>
>> Prelude> let squares = [ x ** 2 | x <- [1 ..] ] :: [Float]
>> Prelude> :print squares
>> squares = (_t1::[Float])
>>
>> You might say that this is more like a generator expression but the
>> result is in fact a list. We can evaluate the first four elements:
>>
>> Prelude> print $ take 4 squares
>> [1.0,4.0,9.0,16.0]
>>
>> And then see that these have been lazily evaluated in the list:
>>
>> Prelude> :print squares
>> squares = 1.0 : 4.0 : 9.0 : 16.0 : (_t2::[Float])
>>
>> A Haskell list comprehension is not a loop at all.
>
> What if you don't take the first four, but instead take just the tenth
> element? Will the preceding elements be calculated too, or do you have
> a sparse list? If the latter, it's not really comparable to a Python
> list, but to some sort of cached mapping from input values to output
> values, which in Python I would implement as a dict with a __missing__
> method. And if the former, well, that's still going to have a loop,
> and it's definitely like a genexp, but genexps have more functionality
> than they do in Python.
The answer is, it depends. If it can avoid evaluating the earlier
elements it will:
Prelude> squares !! 10
121.0
Prelude> :print squares
squares = 1.0 : 4.0 : 9.0 : 16.0 : (_t3::Float) : (_t4::Float) :
(_t5::Float) : (_t6::Float) : (_t7::Float) : (_t8::Float) : 121.0 :
(_t9::[Float])
But sometimes it can't:
Prelude> let triangle = [ (i,j) | i <- [1..], j <- [1..i*i] ]
Prelude> triangle !! 10
(3,6)
Prelude> :print triangle
triangle = (1,1) : (2,1) : (2,2) : (2,3) : (2,4) : (3,1) : (3,2) :
(3,3) : (3,4) : (3,5) : (3,6) : (_t10::[(Integer, Integer)])
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