<html><head><meta http-equiv="content-type" content="text/html; charset=UTF-8"><style>body { line-height: 1.5; }blockquote { margin-top: 0px; margin-bottom: 0px; margin-left: 0.5em; }body { font-size: 10.5pt; font-family: 微软雅黑; color: rgb(0, 0, 0); line-height: 1.5; }</style></head><body>
<div style="orphans: 2; widows: 2; ">This problem also <span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 1.5;">bothered me for a long time, so I write a parse transform to deal with it. To be </span><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 1.5;">specific, say we want to write a function which takes two integers and returns the product and </span><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace">quotient of them.</font></span></div><div style="orphans: 2; widows: 2; "><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace"><br></font></span></div><div style="orphans: 2; widows: 2; "><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal; color: rgb(36, 41, 46);"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">muldiv</span>(<span class="pl-smi" style="box-sizing: border-box;">First</span>, <span class="pl-smi" style="box-sizing: border-box;">Second</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span>
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">case</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">is_integer</span>(<span class="pl-smi" style="box-sizing: border-box;">First</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">of</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">true</span> ->
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">case</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">is_integer</span>(<span class="pl-smi" style="box-sizing: border-box;">Second</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">of</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">true</span> ->
<span class="pl-smi" style="box-sizing: border-box;">Product</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-smi" style="box-sizing: border-box;">First</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">*</span> <span class="pl-smi" style="box-sizing: border-box;">Second</span>,
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">case</span> <span class="pl-smi" style="box-sizing: border-box;">Second</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=/=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">0</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">of</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">true</span> ->
<span class="pl-smi" style="box-sizing: border-box;">Quotient</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-smi" style="box-sizing: border-box;">First</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">div</span> <span class="pl-smi" style="box-sizing: border-box;">Second</span>,
{<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">ok</span>, {<span class="pl-smi" style="box-sizing: border-box;">Product</span>, <span class="pl-smi" style="box-sizing: border-box;">Quotient</span>}};
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">false</span> ->
{<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">error</span>, <span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>Second must not be zero!<span class="pl-pds" style="box-sizing: border-box;">"</span></span>}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">end</span>;
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">false</span> ->
{<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">error</span>, <span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>Second must be an integer!<span class="pl-pds" style="box-sizing: border-box;">"</span></span>}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">end</span>;
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">false</span> ->
{<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">error</span>, <span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>First must be an integer!<span class="pl-pds" style="box-sizing: border-box;">"</span></span>}
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">end</span>.</pre></div><div style="orphans: 2; widows: 2; "><br></div><div style="orphans: 2; widows: 2; ">Ugly, at least I think this "rocket" is not beautiful. Even worse, if requirement changed, you should modify this code on a large scale. To avoid this problem, you can just rewrite your code like this</div><div style="orphans: 2; widows: 2; "><pre style="box-sizing: border-box; font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; font-size: 13.6px; margin-top: 0px; margin-bottom: 0px; word-wrap: normal; padding: 16px; overflow: auto; line-height: 1.45; background-color: rgb(246, 248, 250); border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; word-break: normal; color: rgb(36, 41, 46);"><span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">muldiv</span>(<span class="pl-smi" style="box-sizing: border-box;">First</span>, <span class="pl-smi" style="box-sizing: border-box;">Second</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">-></span>
<span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">do@</span>([<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">esugar_do_transform_error</span> ||
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">case</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">is_integer</span>(<span class="pl-smi" style="box-sizing: border-box;">First</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">of</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">true</span> -> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">return</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">next</span>);
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">false</span> -> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">fail</span>(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>First must be an integer!<span class="pl-pds" style="box-sizing: border-box;">"</span></span>)
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">end</span>,
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">case</span> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">is_integer</span>(<span class="pl-smi" style="box-sizing: border-box;">Second</span>) <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">of</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">true</span> -> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">return</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">next</span>);
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">false</span> -> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">fail</span>(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>Second must be an integer!<span class="pl-pds" style="box-sizing: border-box;">"</span></span>)
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">end</span>,
<span class="pl-smi" style="box-sizing: border-box;">Product</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-smi" style="box-sizing: border-box;">First</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">*</span> <span class="pl-smi" style="box-sizing: border-box;">Second</span>,
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">case</span> <span class="pl-smi" style="box-sizing: border-box;">Second</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=/=</span> <span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">0</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">of</span>
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">true</span> -> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">return</span>(<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">next</span>);
<span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 92, 197);">false</span> -> <span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">fail</span>(<span class="pl-s" style="box-sizing: border-box; color: rgb(3, 47, 98);"><span class="pl-pds" style="box-sizing: border-box;">"</span>Second must not be zero!<span class="pl-pds" style="box-sizing: border-box;">"</span></span>)
<span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">end</span>,
<span class="pl-smi" style="box-sizing: border-box;">Quotient</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">=</span> <span class="pl-smi" style="box-sizing: border-box;">First</span> <span class="pl-k" style="box-sizing: border-box; color: rgb(215, 58, 73);">div</span> <span class="pl-smi" style="box-sizing: border-box;">Second</span>,
<span class="pl-en" style="box-sizing: border-box; color: rgb(111, 66, 193);">return</span>({<span class="pl-smi" style="box-sizing: border-box;">Product</span>, <span class="pl-smi" style="box-sizing: border-box;">Quotient</span>})
]).</pre></div><div style="orphans: 2; widows: 2; "><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace"><br></font></span></div><div style="orphans: 2; widows: 2; "><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace">This idea came from the Haskell's do grammer, and to use this grammer, you just need to write a error monad.</font></span></div><div style="orphans: 2; widows: 2; "><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace"><br></font></span></div><div style="orphans: 2; widows: 2; "><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace">This parse transform code is one module of my esugar(</font></span><span style="font-family: 'SFMono-Regular, Consolas, '; font-size: 10.5pt; line-height: 1.5; background-color: window;">An Erlang Syntactic Sugar Library</span><span style="color: rgb(36, 41, 46); font-family: SFMono-Regular, Consolas, 'Liberation Mono', Menlo, Courier, monospace; background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;">) library, and the address is (</span><span style="font-family: 'SFMono-Regular, Consolas, '; font-size: 10.5pt; line-height: 1.5; background-color: window;"></span><a href="https://github.com/dploop/esugar)" sfmono-regular,="" consolas,="" '";="" font-size:="" 14px;="" font-weight:="" normal;="" font-style:="" normal;'="" style="font-family: ''; font-size: 10.5pt; line-height: 1.5; background-color: window;" class="">https://github.com/dploop/esugar)</a><span sfmono-regular,="" consolas,="" '";="" font-size:="" 14px;="" color:="" rgb(0,="" 0,="" 0);="" background-color:="" rgb(255,="" 255,="" 255);="" font-weight:="" normal;="" font-style:="" normal;text-decoration:="" none;'="" style="font-family: ''; font-size: 10.5pt; line-height: 1.5; background-color: window;"> </span></div><span style="font-family: "" sfmono-regular,="" consolas,="" '";="" font-size:="" 14px;="" color:="" rgb(0,="" 0,="" 0);="" background-color:="" rgb(255,="" 255,="" 255);="" font-weight:="" normal;="" font-style:="" normal;text-decoration:="" none;'=""></span><div style="orphans: 2; widows: 2; "><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace"><br></font></span></div><div style="orphans: 2; widows: 2; "><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace"><span style="line-height: 19px;">I'm glad to discuss with you about how to use it or improve it(because I just write this toy for fun at the begining).</span></font></div><div style="orphans: 2; widows: 2; "><span style="background-color: rgba(0, 0, 0, 0); font-size: 10.5pt; line-height: 19px;"><font color="#24292e" face="SFMono-Regular, Consolas, Liberation Mono, Menlo, Courier, monospace"><br></font></span></div>
<div><br></div><hr style="width: 210px; height: 1px;" color="#b5c4df" size="1" align="left">
<div><span><div style="MARGIN: 10px; FONT-FAMILY: verdana; FONT-SIZE: 10pt"><div>dploop@163.com</div></div></span></div>
<blockquote style="margin-top: 0px; margin-bottom: 0px; margin-left: 0.5em;"><div> </div><div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm"><div style="PADDING-RIGHT: 8px; PADDING-LEFT: 8px; FONT-SIZE: 12px;FONT-FAMILY:tahoma;COLOR:#000000; BACKGROUND: #efefef; PADDING-BOTTOM: 8px; PADDING-TOP: 8px"><div><b>From:</b> <a href="mailto:codewiget95@gmail.com">code wiget</a></div><div><b>Date:</b> 2017-09-25 21:25</div><div><b>To:</b> <a href="mailto:erlang-questions@erlang.org">Erlang-Questions Questions</a></div><div><b>Subject:</b> [erlang-questions] Nested Case Statements v.s. multiple functions</div></div></div><div><div>Hello everyone,</div>
<div> </div>
<div>As I get further into Erlang, I am starting to realize that some of my functions have been getting pretty ugly with nested case statements. For example, I had a nested case statement that looked something like this:</div>
<div> </div>
<div>Send_update(Arg1) -></div>
<div> case do this(Arg1) of</div>
<div> {ok, [Val1, Val2]} -> </div>
<div> case do_that(Val1, Val2) of</div>
<div> {ok, [Val3, Val4]} -></div>
<div> case do_this2(…) of </div>
<div>….</div>
<div> </div>
<div>It continued into this for another few functions, you get the picture - its ugly, and it is hard to read. </div>
<div> </div>
<div>So I went and I converted it to a top level function that would then call lower level functions like so:</div>
<div> </div>
<div> </div>
<div> </div>
<div> send_update(Arg1) -> </div>
<div> case ... of</div>
<div> {ok, [Val1, Val2]} -> </div>
<div> send_update(check_indices, {Arg1, Val1, Val2});</div>
<div> Else -> </div>
<div> lager:error("ERROR: ..")</div>
<div> end.</div>
<div> send_update(check_indices, {Arg1, Arg2, Arg3}) -></div>
<div> case check_indices(Arg2, Arg3)of </div>
<div> true -></div>
<div> send_update(get_values, {Arg1, Arg3});</div>
<div> false -></div>
<div> lager:error("EMERGENCY: ….")</div>
<div> end;</div>
<div> send_update(get_values, {Arg1, Arg2}) -></div>
<div> ...</div>
<div> case ... of </div>
<div> {ok, [Val1, Val2, VAl3]} -></div>
<div> send_update(send_value, {Arg1, Val1, Val2, Val3});</div>
<div> Error -> </div>
<div> lager:error("ERROR: …")</div>
<div> end;</div>
<div> send_update(send_value, {Arg1, Arg2, Arg3, Arg4}) -></div>
<div> …</div>
<div> Do_something(Args),</div>
<div> ok. </div>
<div> </div>
<div> </div>
<div>Now that I look at it though, both don’t look right. They don’t look like something I would write in any other language where I would just have if’s and else’s.</div>
<div> </div>
<div>Is this the proper way to write Erlang? I know everyone has their own style, but I assume there is some accepted form of writing functional programs with deep nests.</div>
<div> </div>
<div>Thank you for your advice!</div>
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</div></blockquote>
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