In crystal Why do I need to do matches.try &.[0]
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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 at 4:49
Cœur
17.3k9102142
asked Nov 20 at 1:37
pitosalas
3,16163363
add a comment |
up vote
0
down vote
favorite
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 at 4:49
Cœur
17.3k9102142
asked Nov 20 at 1:37
pitosalas
3,16163363
add a comment |
up vote
0
down vote
favorite
up vote
0
down vote
favorite
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 at 4:49
Cœur
17.3k9102142
asked Nov 20 at 1:37
pitosalas
3,16163363
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 at 4:49
Cœur
17.3k9102142
asked Nov 20 at 1:37
pitosalas
3,16163363
edited Nov 20 at 4:49
Cœur
17.3k9102142
asked Nov 20 at 1:37
pitosalas
3,16163363
edited Nov 20 at 4:49
Cœur
17.3k9102142
edited Nov 20 at 4:49
Cœur
17.3k9102142
edited Nov 20 at 4:49
Cœur
17.3k9102142
17.3k9102142
asked Nov 20 at 1:37
pitosalas
3,16163363
asked Nov 20 at 1:37
pitosalas
3,16163363
asked Nov 20 at 1:37
pitosalas
3,16163363
3,16163363
add a comment |
add a comment |
1 Answer
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oldest
votes
up vote
4
down vote
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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 at 8:47
Amadan
127k13137190
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
4
down vote
accepted
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 at 8:47
Amadan
127k13137190
add a comment |
up vote
4
down vote
accepted
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 at 8:47
Amadan
127k13137190
add a comment |
up vote
4
down vote
accepted
up vote
4
down vote
accepted
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 at 8:47
Amadan
127k13137190
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 at 8:47
Amadan
127k13137190
edited Nov 20 at 9:06
answered Nov 20 at 8:47
Amadan
127k13137190
answered Nov 20 at 8:47
Amadan
127k13137190
answered Nov 20 at 8:47
Amadan
127k13137190
127k13137190
add a comment |
add a comment |
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