read-ing an octal number
Octal numbers are part of Haskell's integer syntax. Also lex recognizes it:
Prelude> lex "0o10"
[("0o10","")]
But read does not (ghc 8.0.2 on Ubuntu 18.04):
Prelude> read "0o10"
*** Exception: Prelude.read: no parse
The report is not very clear on this in 6.3.3. It just states that lex reads a lexeme and that lex is used by read. Is this intended?
haskell ghci
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
add a comment |
Octal numbers are part of Haskell's integer syntax. Also lex recognizes it:
Prelude> lex "0o10"
[("0o10","")]
But read does not (ghc 8.0.2 on Ubuntu 18.04):
Prelude> read "0o10"
*** Exception: Prelude.read: no parse
The report is not very clear on this in 6.3.3. It just states that lex reads a lexeme and that lex is used by read. Is this intended?
haskell ghci
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
2
That's the same error you would get with any argument toread, because you haven't specified what type of value you want back.
– chepner
Nov 22 '18 at 16:55
add a comment |
Octal numbers are part of Haskell's integer syntax. Also lex recognizes it:
Prelude> lex "0o10"
[("0o10","")]
But read does not (ghc 8.0.2 on Ubuntu 18.04):
Prelude> read "0o10"
*** Exception: Prelude.read: no parse
The report is not very clear on this in 6.3.3. It just states that lex reads a lexeme and that lex is used by read. Is this intended?
haskell ghci
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
Octal numbers are part of Haskell's integer syntax. Also lex recognizes it:
Prelude> lex "0o10"
[("0o10","")]
But read does not (ghc 8.0.2 on Ubuntu 18.04):
Prelude> read "0o10"
*** Exception: Prelude.read: no parse
The report is not very clear on this in 6.3.3. It just states that lex reads a lexeme and that lex is used by read. Is this intended?
haskell ghci
haskell ghci
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
asked Nov 22 '18 at 13:06
falsefalse
10.3k771144
10.3k771144
2
That's the same error you would get with any argument toread, because you haven't specified what type of value you want back.
– chepner
Nov 22 '18 at 16:55
add a comment |
2
That's the same error you would get with any argument toread, because you haven't specified what type of value you want back.
– chepner
Nov 22 '18 at 16:55
2
2
That's the same error you would get with any argument to
read, because you haven't specified what type of value you want back.– chepner
Nov 22 '18 at 16:55
That's the same error you would get with any argument to
read, because you haven't specified what type of value you want back.– chepner
Nov 22 '18 at 16:55
add a comment |
1 Answer
1
active
oldest
votes
The type of read "0o10" is:
GHCi> :t read "0o10"
read "0o10" :: Read a => a
In GHCi, the type variable a will be defaulted to (), as explained in the User's Guide:
GHCi> read "()"
()
Note that the behaviour you describe is not limited to strings that would be parsed as octals...
GHCi> read "[1,2,3]"
*** Exception: Prelude.read: no parse
... and that lex :: ReadS String, unlike read, specifically produces String results, rather than allowing you to pick an instance of Read.
To get the behaviour you expect, provide a suitable type annotation or signature:
GHCi> read "0o10" :: Integer
8
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
add a comment |
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1 Answer
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1 Answer
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votes
The type of read "0o10" is:
GHCi> :t read "0o10"
read "0o10" :: Read a => a
In GHCi, the type variable a will be defaulted to (), as explained in the User's Guide:
GHCi> read "()"
()
Note that the behaviour you describe is not limited to strings that would be parsed as octals...
GHCi> read "[1,2,3]"
*** Exception: Prelude.read: no parse
... and that lex :: ReadS String, unlike read, specifically produces String results, rather than allowing you to pick an instance of Read.
To get the behaviour you expect, provide a suitable type annotation or signature:
GHCi> read "0o10" :: Integer
8
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
add a comment |
The type of read "0o10" is:
GHCi> :t read "0o10"
read "0o10" :: Read a => a
In GHCi, the type variable a will be defaulted to (), as explained in the User's Guide:
GHCi> read "()"
()
Note that the behaviour you describe is not limited to strings that would be parsed as octals...
GHCi> read "[1,2,3]"
*** Exception: Prelude.read: no parse
... and that lex :: ReadS String, unlike read, specifically produces String results, rather than allowing you to pick an instance of Read.
To get the behaviour you expect, provide a suitable type annotation or signature:
GHCi> read "0o10" :: Integer
8
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
add a comment |
The type of read "0o10" is:
GHCi> :t read "0o10"
read "0o10" :: Read a => a
In GHCi, the type variable a will be defaulted to (), as explained in the User's Guide:
GHCi> read "()"
()
Note that the behaviour you describe is not limited to strings that would be parsed as octals...
GHCi> read "[1,2,3]"
*** Exception: Prelude.read: no parse
... and that lex :: ReadS String, unlike read, specifically produces String results, rather than allowing you to pick an instance of Read.
To get the behaviour you expect, provide a suitable type annotation or signature:
GHCi> read "0o10" :: Integer
8
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
The type of read "0o10" is:
GHCi> :t read "0o10"
read "0o10" :: Read a => a
In GHCi, the type variable a will be defaulted to (), as explained in the User's Guide:
GHCi> read "()"
()
Note that the behaviour you describe is not limited to strings that would be parsed as octals...
GHCi> read "[1,2,3]"
*** Exception: Prelude.read: no parse
... and that lex :: ReadS String, unlike read, specifically produces String results, rather than allowing you to pick an instance of Read.
To get the behaviour you expect, provide a suitable type annotation or signature:
GHCi> read "0o10" :: Integer
8
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
edited Nov 22 '18 at 13:49
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
answered Nov 22 '18 at 13:20
duplodeduplode
22.9k44885
22.9k44885
add a comment |
add a comment |
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2
That's the same error you would get with any argument to
read, because you haven't specified what type of value you want back.– chepner
Nov 22 '18 at 16:55