Idris是否有非终止条款？

Question

关于官方伊德里斯维基的非官方常见问题解答（官方称其为语言＆＃39; s git repo），它是stated that

  

总体语言[例如Idris]我们没有未定义和非终止条款   所以我们不用担心评估它们。

但是，ones的以下定义（使用List而不是Stream）肯定似乎没有终止：

ones: List Int
ones = 1 :: ones
-- ...
printLn(head ones) -- seg fault!

所以，我不确定wiki条目是否错误，或者我是否误解了上下文。请注意Stream解决方法已经described in the Idris tutorial。

Answer 1

如果你认为伊德里斯是完全的，那么这只是总数。您可以编写%default total，%default covering或%default partial（默认值）之一，之后的所有声明都将采用给定的整体注释：

%default total

-- implicitly total
ones1 : List Int
ones1 = 1 :: ones1
-- ERROR: ones1 is not total

-- total
ZNeverHeardOfIt : Nat -> Nat
ZNeverHeardOfIt (S n) = n
-- ERROR: missing cases in ZNeverHeardOfIt

covering
natRoulette : Nat -> Nat
natRoulette Z = Z
natRoulette (S n) = natRoulette (S (S n))
-- covering means all possible inputs are covered by an equation
-- but the function isn't checked for termination
-- natRoulette has cases for all inputs, but it might go into an infinite loop
-- it's morally equivalent to just partial, as a function that loops forever
-- on an input isn’t very different from one missing the case
-- it just gets the compiler to complain more

partial
ones : List Int
ones = 1 :: ones
-- no checks at all
-- Idris, being strict, needs to evaluate ones strictly before it can evaluate ones.
-- Oh wait, that's impossible. Idris promptly vanishes in a segfault.