What does `|` mean in a goal-type in Agda?

Question

I'm reading the Brutal Meta-introduction to Agda.

In the section on "Rewriting with with and Unification" they mention a a case where a type of a goal goes from:

(filter p (a ∷ as) | p a) ≡ (filterN p (a ∷ as) | p a)

to

(filter p (a ∷ rs) | r) ≡ (filterN p (a ∷ rs) | r)

after adding a with clause.

I've seen similar notation show up in goals and error messages when writing Agda code.

I'm wondering, does it mean to have a variable to the right of |, in this notation? Is this documented anywhere?

Answer 1

如果你看一下过滤器的定义，你会看到像

这样的子句

... | true  = a ∷ (filter p as)

是

的简写

filter p (a ∷ as) | true = a ∷ (filter p as)

|目标中的符号表示这些条款，这意味着例如

filter p (a ∷ as) | e

等于＆＃34;a∷（滤波器p as）＆＃34;当＆＃34; e＆＃34;等于＆＃34; true＆＃34;。

在你的情况下，你有一个变量＆＃39; r＆＃39;，所以如果你对它进行模式匹配，事情会计算得更多。