<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Dear Cafe,<div class=""><br class=""></div><div class="">I find myself unable to reason about an error that `optional` in a parser err out instead of return Nothing, I asked the question at StackOverflow, and would like to seek your help here as well.</div><div class=""><br class=""></div><div class=""><a href="https://stackoverflow.com/questions/64522568/why-optional-in-a-parser-can-err-out" class="">https://stackoverflow.com/questions/64522568/why-optional-in-a-parser-can-err-out</a></div><div class=""><br class=""></div><div class=""><div style="margin: 0px 0px var(--s-prose-spacing); padding: 0px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: Arial, "Helvetica Neue", Helvetica, sans-serif; font-size: 15px; vertical-align: baseline; box-sizing: inherit; clear: both; color: rgb(218, 215, 210); orphans: 2; widows: 2; background-color: rgb(34, 37, 38);" class=""><a href="https://github.com/complyue/dcp" rel="nofollow noreferrer" style="color: rgb(118, 193, 247); margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; cursor: pointer;" class="">https://github.com/complyue/dcp</a> is a minimum working example to reprod this error</div><pre style="margin-top: 0px; margin-bottom: calc(var(--s-prose-spacing) + 0.4em); padding: 12px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: 1.30769; font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace, sans-serif; font-size: 13px; vertical-align: baseline; box-sizing: inherit; width: auto; max-height: 600px; overflow: auto; background-color: rgb(21, 22, 23); border-top-left-radius: 5px; border-top-right-radius: 5px; border-bottom-right-radius: 5px; border-bottom-left-radius: 5px; overflow-wrap: normal; color: rgb(232, 230, 227); orphans: 2; widows: 2;" class=""><code style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace, sans-serif; vertical-align: baseline; box-sizing: inherit; background-color: transparent; white-space: inherit; color: rgb(218, 215, 210); border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px;" class="">$ cabal run dcp:dcp < samples/basic.txt
Up to date
dcp: 10:1:
|
10 | method doXXX() pass
| ^
unexpected 'm'
expecting ';'
CallStack (from HasCallStack):
error, called at src/Parser.hs:149:14 in main:Parser
$
</code></pre><div style="margin: 0px 0px var(--s-prose-spacing); padding: 0px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: Arial, "Helvetica Neue", Helvetica, sans-serif; font-size: 15px; vertical-align: baseline; box-sizing: inherit; clear: both; color: rgb(218, 215, 210); orphans: 2; widows: 2; background-color: rgb(34, 37, 38);" class="">I believe it's <code style="margin: 0px; padding: 2px 4px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace, sans-serif; font-size: 13px; vertical-align: baseline; box-sizing: inherit; background-color: rgb(50, 54, 56); white-space: pre-wrap; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px;" class="">optionalSemicolon</code> causing the failure:</div><div style="margin: 0px 0px var(--s-prose-spacing); padding: 0px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: Arial, "Helvetica Neue", Helvetica, sans-serif; font-size: 15px; vertical-align: baseline; box-sizing: inherit; clear: both; color: rgb(218, 215, 210); orphans: 2; widows: 2; background-color: rgb(34, 37, 38);" class=""><a href="https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L50-L54" rel="nofollow noreferrer" style="color: rgb(118, 193, 247); margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; cursor: pointer;" class="">https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L50-L54</a></div><pre class="s-code-block lang-hs hljs haskell" style="margin-top: 0px; margin-bottom: calc(var(--s-prose-spacing) + 0.4em); padding: 12px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: 1.30769; font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace, sans-serif; font-size: 13px; vertical-align: baseline; box-sizing: inherit; width: auto; max-height: 600px; overflow: auto; background-color: rgb(21, 22, 23); border-top-left-radius: 5px; border-top-right-radius: 5px; border-bottom-right-radius: 5px; border-bottom-left-radius: 5px; color: rgb(232, 230, 227); overflow-wrap: normal; orphans: 2; widows: 2;"><code style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; background-color: transparent; white-space: inherit;" class=""> findIt = <span class="hljs-keyword" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(136, 175, 206);">do</span>
<span class="hljs-comment" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(168, 160, 149);">-- ignore leading whitespaces and an optional semicolon in between</span>
nbsc >> optionalSemicolon >> nbsc
<span class="hljs-comment" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(168, 160, 149);">-- try get a doc comment block</span>
getIt >>= \<span class="hljs-keyword" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(136, 175, 206);">case</span>
</code></pre><div style="margin: 0px 0px var(--s-prose-spacing); padding: 0px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: Arial, "Helvetica Neue", Helvetica, sans-serif; font-size: 15px; vertical-align: baseline; box-sizing: inherit; clear: both; color: rgb(218, 215, 210); orphans: 2; widows: 2; background-color: rgb(34, 37, 38);" class="">And it's defined like this: <a href="https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L31-L32" rel="nofollow noreferrer" style="color: rgb(118, 193, 247); margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; cursor: pointer;" class="">https://github.com/complyue/dcp/blob/1df7ad590d78d4fa9a017eb53f9f265e291bdfa7/src/Parser.hs#L31-L32</a></div><pre class="s-code-block lang-hs hljs haskell" style="margin-top: 0px; margin-bottom: calc(var(--s-prose-spacing) + 0.4em); padding: 12px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: 1.30769; font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace, sans-serif; font-size: 13px; vertical-align: baseline; box-sizing: inherit; width: auto; max-height: 600px; overflow: auto; background-color: rgb(21, 22, 23); border-top-left-radius: 5px; border-top-right-radius: 5px; border-bottom-right-radius: 5px; border-bottom-left-radius: 5px; color: rgb(232, 230, 227); overflow-wrap: normal; orphans: 2; widows: 2;"><code style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; background-color: transparent; white-space: inherit;" class=""><span class="hljs-title" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(241, 146, 80);">optionalSemicolon</span> :: <span class="hljs-type" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(241, 146, 80);">Parser</span> <span class="hljs-type" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(241, 146, 80);">Bool</span>
<span class="hljs-title" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(241, 146, 80);">optionalSemicolon</span> = fromMaybe <span class="hljs-type" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(241, 146, 80);">False</span> <$> optional (<span class="hljs-type" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(241, 146, 80);">True</span> <$ symbol <span class="hljs-string" style="margin: 0px; padding: 0px; border: 0px; font-style: inherit; font-variant-ligatures: inherit; font-variant-position: inherit; font-variant-caps: inherit; font-variant-numeric: inherit; font-variant-alternates: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: baseline; box-sizing: inherit; color: rgb(186, 193, 114);">";"</span>)
</code></pre><div style="margin: 0px; padding: 0px; border: 0px; font-variant-ligatures: normal; font-variant-numeric: inherit; font-variant-east-asian: inherit; font-stretch: inherit; line-height: inherit; font-family: Arial, "Helvetica Neue", Helvetica, sans-serif; font-size: 15px; vertical-align: baseline; box-sizing: inherit; clear: both; color: rgb(218, 215, 210); orphans: 2; widows: 2; background-color: rgb(34, 37, 38);" class="">I can't reason about why it can fail like this.</div></div><div class=""><br class=""></div><div class="">Best regards,</div><div class="">Compl</div><div class=""><br class=""></div></body></html>