erlang - Erlang中列表理解中的多个过滤器

Question

假设我有一个包含天气的列表：

1> Weather = [{toronto, rain}, {montreal, storms}, {london, fog}, 
    {paris, sun}, {boston, fog}, {vancouver, snow}].

为了得到有雾的地方，我可以这样做：

2> FoggyPlaces = [X || {X, fog} <- Weather].
[london,boston]

现在我想检索既有雾又有雪的地方。我试过这个，但它只检索下雪的地方，

3> FoggyAndSnowyPlaces = [X || {X, fog} <- Weather, {X,snow} <- Weather].
[vancouver,vancouver]

我期待的地方[london,boston,vancouver]。

如何包含多个过滤器？

Answer 1

FoggyAndSnowyPlaces = [X || {X, Y} <- Weather, (Y == fog) or (Y == snow)].

You are confusing generators (Pattern <- List) and filters (boolean conditions). Multiple generators work like nested loops in other languages, so in your 3> you get vancouver twice because the first generator produces two values.