read-ing an octal number












2















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?










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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


















2















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?










share|improve this question


















  • 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
















2












2








2








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?










share|improve this question














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






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asked Nov 22 '18 at 13:06









falsefalse

10.3k773151




10.3k773151








  • 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
















  • 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










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














1 Answer
1






active

oldest

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7














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





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    1 Answer
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    7














    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





    share|improve this answer






























      7














      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





      share|improve this answer




























        7












        7








        7







        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





        share|improve this answer















        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






        share|improve this answer














        share|improve this answer



        share|improve this answer








        edited Nov 22 '18 at 13:49

























        answered Nov 22 '18 at 13:20









        duplodeduplode

        23.1k44987




        23.1k44987
































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