haskell - 使用 map 和 concat 将列表推导式转换为定义

Question

摘自用 Haskell 进行函数式思考，第 67 页:

[...] list comprehensions are translated into equivalent definitions in
terms of map and concat. The translation rules are:

[e | True]       = [e] 
[e | q]          = [e | q, True] 
[e | b, Q]       = if b then [e | Q] else [] 
[e | p <- xs, Q] = let ok p = [e | Q]
                       ok _ = []
                   in concat (map ok xs)

作者没有定义 e、q、Q 或 b。我认为第一个意思是“表达”，但我以前从未见过其他的。有人可以启发我吗？

Answer 1

此翻译直接来自官方Haskell Report ，其中有这样的附加句子:

where e ranges over expressions, p over patterns, l over list-valued expressions, b over boolean expressions, decls over declaration lists, q over qualifiers, and Q over sequences of qualifiers. ok is a fresh variable. The function concatMap, and boolean value True, are defined in the Prelude.

如果您想知道这些术语的含义，该报告提供了更多详细信息。