Haskell, range downto without step [duplicate]

问题:

This question already has an answer here:

• Decrementing ranges in Haskell 3 answers

Why in Haskell is not working range downto without step

[7..1] => []

but working only this

[7,6..1] => [7,6,5,4,3,2,1] 

回答1:

3.10. Arithmetic sequences

[...] Arithmetic sequences satisfy these identities:

• [...]
• [ e1..e3 ] = enumFromTo e1 e3
• [...]

6.3.4 The Enum Class

For the types Int and Integer, the enumeration functions have the following meaning:

• [...]
• The sequence enumFromTo e1 e3 is the list [e1,e1 + 1,e1 + 2,…e3]. The list is empty if e1 > e3.
• [...]

From Haskell 2010 Language Report.

回答2:

Haskell has no way to know that you want to step -1 until you give it a hint.

There might be situations where you want a range [x..y] where y < x and where you expect the range to be empty. This would create subtle bugs if Haskell would simply step downwards in these cases.

回答3:

Without an indication of the step, haskell assumes it to be +1 and returns an empty list if it's not applicable to the given parameters.

Any increment apart from +1 has to be explicitly suggested; not only positive integrers > 1.