In crystal Why do I need to do matches.try &.[0]
Here's some code:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address) # line 1
puts matches[0]? # line 2
puts matches.try &.[0] # line 3
matches datatype is (Regex::MatchData | Nil) according to typeof. Yet, line 2, referring to matches[0] fails with a compilation error. And I don't understand line 3 at all!
Could someone clarify?
crystal-lang
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
add a comment |
Here's some code:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address) # line 1
puts matches[0]? # line 2
puts matches.try &.[0] # line 3
matches datatype is (Regex::MatchData | Nil) according to typeof. Yet, line 2, referring to matches[0] fails with a compilation error. And I don't understand line 3 at all!
Could someone clarify?
crystal-lang
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
add a comment |
Here's some code:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address) # line 1
puts matches[0]? # line 2
puts matches.try &.[0] # line 3
matches datatype is (Regex::MatchData | Nil) according to typeof. Yet, line 2, referring to matches[0] fails with a compilation error. And I don't understand line 3 at all!
Could someone clarify?
crystal-lang
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
Here's some code:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address) # line 1
puts matches[0]? # line 2
puts matches.try &.[0] # line 3
matches datatype is (Regex::MatchData | Nil) according to typeof. Yet, line 2, referring to matches[0] fails with a compilation error. And I don't understand line 3 at all!
Could someone clarify?
crystal-lang
crystal-lang
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
edited Nov 20 '18 at 4:49
Cœur
18.2k9108148
18.2k9108148
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
asked Nov 20 '18 at 1:37
pitosalaspitosalas
3,24063365
3,24063365
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
Line 2: As you say, the type of matches is (Regex::MatchData | Nil). In case it happens to be nil, it wouldn't have the #? method, which makes the type checker angry. You are supposed to check whether match succeeded first:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address)
if matches
puts matches[0]?
end
Inside if, the type of matches is just Regex::MatchData (as we eliminated the Nil possibility), and the type checker can rest peacefully.
If you are sure your string will match, you can pacify the type checker with not_nil!, but that opens up a possibility of a runtime error, if your confidence in your data's conformance was unfounded:
puts matches.not_nil![0]?
Line 3: #try will perform the block, except when the caller is nil, when it returns nil. No guards are needed because #try is explicitly defined on Nil (as well as on Object).
It uses the shortcut syntax for blocks, where &.[0] is kind of equivalent to { |x| x[0] }.
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53385025%2fin-crystal-why-do-i-need-to-do-matches-try-0%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Line 2: As you say, the type of matches is (Regex::MatchData | Nil). In case it happens to be nil, it wouldn't have the #? method, which makes the type checker angry. You are supposed to check whether match succeeded first:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address)
if matches
puts matches[0]?
end
Inside if, the type of matches is just Regex::MatchData (as we eliminated the Nil possibility), and the type checker can rest peacefully.
If you are sure your string will match, you can pacify the type checker with not_nil!, but that opens up a possibility of a runtime error, if your confidence in your data's conformance was unfounded:
puts matches.not_nil![0]?
Line 3: #try will perform the block, except when the caller is nil, when it returns nil. No guards are needed because #try is explicitly defined on Nil (as well as on Object).
It uses the shortcut syntax for blocks, where &.[0] is kind of equivalent to { |x| x[0] }.
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
add a comment |
Line 2: As you say, the type of matches is (Regex::MatchData | Nil). In case it happens to be nil, it wouldn't have the #? method, which makes the type checker angry. You are supposed to check whether match succeeded first:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address)
if matches
puts matches[0]?
end
Inside if, the type of matches is just Regex::MatchData (as we eliminated the Nil possibility), and the type checker can rest peacefully.
If you are sure your string will match, you can pacify the type checker with not_nil!, but that opens up a possibility of a runtime error, if your confidence in your data's conformance was unfounded:
puts matches.not_nil![0]?
Line 3: #try will perform the block, except when the caller is nil, when it returns nil. No guards are needed because #try is explicitly defined on Nil (as well as on Object).
It uses the shortcut syntax for blocks, where &.[0] is kind of equivalent to { |x| x[0] }.
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
add a comment |
Line 2: As you say, the type of matches is (Regex::MatchData | Nil). In case it happens to be nil, it wouldn't have the #? method, which makes the type checker angry. You are supposed to check whether match succeeded first:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address)
if matches
puts matches[0]?
end
Inside if, the type of matches is just Regex::MatchData (as we eliminated the Nil possibility), and the type checker can rest peacefully.
If you are sure your string will match, you can pacify the type checker with not_nil!, but that opens up a possibility of a runtime error, if your confidence in your data's conformance was unfounded:
puts matches.not_nil![0]?
Line 3: #try will perform the block, except when the caller is nil, when it returns nil. No guards are needed because #try is explicitly defined on Nil (as well as on Object).
It uses the shortcut syntax for blocks, where &.[0] is kind of equivalent to { |x| x[0] }.
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
Line 2: As you say, the type of matches is (Regex::MatchData | Nil). In case it happens to be nil, it wouldn't have the #? method, which makes the type checker angry. You are supposed to check whether match succeeded first:
matches = //([a-z]+)/(d+)/state/([a-z]+)/.match(address)
if matches
puts matches[0]?
end
Inside if, the type of matches is just Regex::MatchData (as we eliminated the Nil possibility), and the type checker can rest peacefully.
If you are sure your string will match, you can pacify the type checker with not_nil!, but that opens up a possibility of a runtime error, if your confidence in your data's conformance was unfounded:
puts matches.not_nil![0]?
Line 3: #try will perform the block, except when the caller is nil, when it returns nil. No guards are needed because #try is explicitly defined on Nil (as well as on Object).
It uses the shortcut syntax for blocks, where &.[0] is kind of equivalent to { |x| x[0] }.
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
edited Nov 20 '18 at 9:06
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
answered Nov 20 '18 at 8:47
AmadanAmadan
130k13143195
130k13143195
add a comment |
add a comment |
Thanks for contributing an answer to Stack Overflow!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53385025%2fin-crystal-why-do-i-need-to-do-matches-try-0%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
