OS: MacOSX 10.7.1 GHC and Haskell-platform from brew.
GHCi, version 7.0.4: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
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Loading package ffi-1.0 ... linking ... done.
Prelude> :m +Text.Regex.Posix
Prelude Text.Regex.Posix> "foo" =~ "o" :: [String]
<interactive>:1:7:
No instance for (RegexContext Regex [Char] [String])
arising from a use of `=~'
Possible fix:
add an instance declaration for
(RegexContext Regex [Char] [String])
In the expression: "foo" =~ "o" :: [String]
In an equation for `it': it = "foo" =~ "o" :: [String]
Prelude Text.Regex.Posix> "foo" =~ "o" :: String
Loading package array-0.3.0.2 ... linking ... done.
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"o"
I believe libraries updated. And I think the output of "foo" =~ "o" :: [String]
is ["o", "o"]
Any suggestion will be appreciate.
ghci> getAllTextMatches ("foo" =~ "o" :: AllTextMatches [] String)
["o", "o"]
I haven't used regexes in Haskell much (which is I think Dan Burton's answer is more idiomatic).
So the way I figured this out is I looked at your type error No instance for (RegexContext Regex [Char] [String])
, and popped into ghci:
ghci> :t (=~)
(=~)
:: (RegexMaker Regex CompOption ExecOption source,
RegexContext Regex source1 target) =>
source1 -> source -> target
So RegexContext Regex [Char] [String]
is a class that includes the return type of "foo" =~ "o" :: [String]
. So I looked to see what instances of this class did exist, so I could find out what the return value was allowed to be:
ghci> :i RegexContext
class RegexLike
regex source => RegexContext regex source target where
match :: regex -> source -> target
matchM :: Monad m => regex -> source -> m target
-- Defined in Text.Regex.Base.RegexLike
instance RegexContext Regex String String
-- Defined in Text.Regex.Posix.String
instance RegexLike a b => RegexContext a b [[b]]
-- Defined in Text.Regex.Base.Context
...
instance RegexLike a b => RegexContext a b (AllTextMatches [] b)
-- Defined in Text.Regex.Base.Context
...
The AllTextMatches
name seemed to indicate what you were looking for, so I checked that out:
ghci> :i AllTextMatches
newtype AllTextMatches f b
= AllTextMatches {getAllTextMatches :: f b}
-- Defined in Text.Regex.Base.RegexLike
instance RegexLike a b => RegexContext a b (AllTextMatches [] b)
-- Defined in Text.Regex.Base.Context
So this was the type to use to extract all the text matches, as I suspected. All I needed to do was indicate that I wanted a return value of that type.
Note also the possible return type of [[b]]
, which I assume returns a list of lists containing each complete match and all its submatches:
ghci> "foo" =~ "o" :: [[String]]
[["o"],["o"]]
ghci> "foo bar baz" =~ "[aeiou](.)" :: [[String]]
[["oo","o"],["ar","r"],["az","z"]]
So maybe that's the type you meant to use, instead of [String]
. I could see [String]
as being slightly ambiguous when [[String]]
existed - should "foo bar baz" =~ "[aeiou](.)" :: [String]
be fst ("foo bar baz" =~ "[aeiou](.)" :: [[String]])
or map fst ("foo bar baz" =~ "[aeiou](.)" :: [[String]])
.
This also works
ghci> getAllTextMatches $ "foo" =~ "o" :: [String]
["o","o"]
Try this:
Prelude Text.Regex.Posix> getAllTextMatches ("foo" =~ "o" :: AllTextMatches [] String)
["o","o"]
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