elm - Elm 中至少有两个元素的列表上的模式匹配

Question

以下模式匹配至少有两个元素的列表有什么问题pos1和 pos2 ?

type Pos = (Float, Float)
type Tail = [Pos]

tail_cut : Float -> Tail -> Tail
tail_cut _ [] = []
tail_cut _ [pos] = [pos]
tail_cut cut (pos1:pos2:poss) = []   --line 91

[1 of 1] Compiling Main
Parse error at (line 91, column 19):
unexpected ":"
expecting "::", pattern, whitespace, comma ',' or closing paren ')'

请注意，我没有发布正文，只是返回一个空列表以保持片段较小。

Answer 1

在榆树，cons定义为 ::而不是 :
见:http://library.elm-lang.org/catalog/elm-lang-Elm/0.13/List

应该这样做:(pos1::pos2::poss)