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How to reference captured groups in the replacement? #2

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opened 2023-05-09 03:48:08 +02:00 by halloleo · 3 comments

halloleo commented

2023-05-09 03:48:08 +02:00

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I try t use a captured group in the repalcement. For example

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes, FlexibleContexts #-}

import Text.Regex.PCRE.Heavy as RE

cleanMe  :: Text -> Text
cleanMe txt = RE.gsub [re|^([0-9]*)DELETABLE-NUMMARKER]  "\1NEW-NUMMARKER" txt

But this does not the work. the only way I made it works is the cumbdersome function approach:

cleanMe txt = RE.gsub [re|^(.*)DELETABLE-MARKER]  (\(g1:_) -> g1 ++ "NEW-NUMMARKER") txt

How can I reference the captures group directly in the replacement argument?

I try t use a captured group in the repalcement. For example ``` {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE QuasiQuotes, FlexibleContexts #-} import Text.Regex.PCRE.Heavy as RE cleanMe :: Text -> Text cleanMe txt = RE.gsub [re|^([0-9]*)DELETABLE-NUMMARKER] "\1NEW-NUMMARKER" txt ``` But this does not the work. the only way I made it works is the cumbdersome function approach: ``` cleanMe txt = RE.gsub [re|^(.*)DELETABLE-MARKER] (\(g1:_) -> g1 ++ "NEW-NUMMARKER") txt ``` How can I reference the captures group directly in the replacement argument?

halloleo changed title from ~~How to add caltured groups in teh repalve,ent?~~ to How to reference captured groups in the replacement?

2023-05-09 03:48:45 +02:00

valpackett commented

2023-05-09 05:13:27 +02:00

Owner

Yes, you can only do it with functions. I haven't found it to be cumbersome and I rather like not having another stringy syntax.

I guess it would be nice to have a TH thingy that would compile e.g. [rep|\1NEW-NUMMARKER|] to (\(g1:_) -> g1 ++ "NEW-NUMMARKER"). PRs welcome :)

Yes, you can only do it with functions. I haven't found it to be cumbersome and I rather like not having another stringy syntax. I guess it would be nice to have a TH thingy that would compile e.g. `[rep|\1NEW-NUMMARKER|]` to `(\(g1:_) -> g1 ++ "NEW-NUMMARKER")`. PRs welcome :)

halloleo commented

2023-05-10 04:39:28 +02:00

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Contributor

Thanks for the clarification!

Using a function is not too bad, but I always get these "Pattern match(es) are non-exhaustive In a lambda abstraction" warning on that function, which is annoying...

I am just starting out with Haskell, so I don't think I can write appropriate code (yet), but I certainly can update the documentation to make it clearer that functions are the only way to get captured groups into the replacement.

Thanks for the clarification! Using a function is not *too* bad, but I always get these "_Pattern match(es) are non-exhaustive In a lambda abstraction_" warning on that function, which is annoying... I am just starting out with Haskell, so I don't think I can write appropriate code (yet), but I certainly can update the documentation to make it clearer that functions are the _only_ way to get captured groups into the replacement.

halloleo referenced this issue

2023-05-15 09:48:32 +02:00

Add note about captured groups and replacements #3

bdesham commented

2023-08-30 04:22:06 +02:00

Using a function is not too bad, but I always get these "Pattern match(es) are non-exhaustive In a lambda abstraction" warning on that function, which is annoying...

FYI, you could also write your example like

cleanMe txt = RE.gsub [re|^(.*)DELETABLE-MARKER] replace txt
  where
    replace :: [String] -> String
    replace [g1] = g1 <> "NEW-NUMMARKER"
    replace _ = error "replace function received the wrong number of arguments"

This still has the underlying problem that you were getting the warning about—that your replacement function is expecting a list with exactly one element, and that it will crash if it gets a different number of elements—but it’s more explicit about this failure case (because of the explicit use of error) and it should also suppress that warning.

> Using a function is not too bad, but I always get these "Pattern match(es) are non-exhaustive In a lambda abstraction" warning on that function, which is annoying... FYI, you could also write your example like ```haskell cleanMe txt = RE.gsub [re|^(.*)DELETABLE-MARKER] replace txt where replace :: [String] -> String replace [g1] = g1 <> "NEW-NUMMARKER" replace _ = error "replace function received the wrong number of arguments" ``` This still has the underlying problem that you were getting the warning about—that your replacement function is expecting a list with exactly one element, and that it will crash if it gets a different number of elements—but it’s more explicit about this failure case (because of the explicit use of `error`) and it should also suppress that warning.

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