From: Jim Date: Sun, 1 Feb 2015 18:21:18 +0000 (-0500) Subject: tweak week1 advanced X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=536b3ca117f2fbd3fecc229e62a63a41294428d3 tweak week1 advanced --- diff --git a/topics/week1_advanced_notes.mdwn b/topics/week1_advanced_notes.mdwn index 1f58f61a..2742a7fc 100644 --- a/topics/week1_advanced_notes.mdwn +++ b/topics/week1_advanced_notes.mdwn @@ -101,7 +101,7 @@ If we get to the `y & ys` line in the pattern list, and the pattern-match succee Sometimes it's useful to bind variables against overlapping parts of a structure. For instance, suppose I'm writing a pattern that is to be matched against multivalues like `([10, 20], 'true)`. And suppose I want to end up with `ys` bound to `[10, 20]`, `x` bound to `10`, and `xs` bound to `[20]`. Using the techniques introduced so far, I have two options. First, I could bind `ys` against `[10, 20]`, and then initiate a second pattern-match to break that up into `10` and `[20]`. Like this: - case [10, 20] of + case ([10, 20], 'true) of [ys, _] then case ys of x & xs then ...; ... @@ -111,7 +111,7 @@ Sometimes it's useful to bind variables against overlapping parts of a structure Alternatively, I could directly bind `x` against `10` and `xs` against `[20]`. But then I would have to re-cons them together again to get `ys`. Like this: - case [10, 20] of + case ([10, 20], 'true) of [x & xs, _] then let ys match x & xs in ...; @@ -120,7 +120,7 @@ Alternatively, I could directly bind `x` against `10` and `xs` against `[20]`. B Both of these strategies work. But they are a bit inefficient. I said you didn't really need to worry about efficiency in this seminar. But these are also a bit cumbersome to write. There's a special syntax that enables us to bind all three of `ys`, `x`, and `xs` in the desired way, despite the fact that they will be matching against overlapping, rather than discrete, parts of the value `[10, 20]`. The special syntax looks like this: - case [10, 20] of + case ([10, 20], 'true) of [(x & xs) as ys, _] then ... ... end