Provide a pure version of `input`

[lthms]

Hi! I am currently working on a project whichin I would love to use dhall to improve its customization power. However, in order to do so without breaking too many things, I was wondering if it would be possible to use dhall script in pure functions.

That is, I would love to be able to load and parse the script once, then run it in a pure way several time with different arguments.

Do you think it is possible?

[Gabriel439]

The next release of dhall will expose the internals of the Type constructor: 0221dd4

... so you will have access to a new extract function of type:

extract :: Type a -> Expr X X -> Maybe a

... and if you apply that to auto, it would have type:

extract auto :: Interpret a => Expr X X -> Maybe a

... which gives you a pure extraction function. Would that satisfy your use case?

[lthms]

That sounds great indeed! What if my dhall script is a function, lets say \(str: Text) -> ... which returns a Text. My understanding is that currently dhall does not let you just use input on the script without an argument, as there is no Interpret instance for Text -> Text. Will there be such an instance in the next release? Or, do you think such an instance could be written? I would be happy to help.

[Gabriel439]

Assuming that you have a Dhall function of type Text → Text encoded in Haskell as a value of the following type:

myDhallFunction :: Expr X X

You can turn that into a Haskell function of type Text -> Text like this:

myHaskellFunction :: Text -> Text
myHaskellFunction input = case extract auto (App myDhallFunction (TextLit input)) of
    Just input -> input
    Nothing    -> error "This shouldn't happen if the function type-checks"

I can add an interpret instance that does this for a limited subset of functions but it will take some time because it requires adding a new class, so I would recommend use the above approach for now

[Gabriel439]

Sorry, one correction. It should be:

myHaskellFunction :: Text -> Text
myHaskellFunction input = case extract auto (normalize (App myDhallFunction (TextLit input))) of
    Just input -> input
    Nothing    -> error "This shouldn't happen if the function type-checks"

[Gabriel439]

Here's a first draft of what I have in mind: #88

It still needs more work though (to add tests, documentation, and generic programming support for user-defined types)

[lthms]

Thanks a lot @Gabriel439!

I need some time to process that, as I am not very familiar with the dhall package interface. Is the Dhall.Tutorial up-to-date with the latest changes you’ve made?

[Gabriel439]

The tutorial doesn't explain how to use this yet, but here's an example:

>>> :set -XOverloadedStrings
>>> f <- input auto "λ(x : Text) → λ(y : Text) → x ++ y" :: IO (Text -> Text -> Text)
>>> f "ABC" "DEF"
"ABCDEF"

[lthms]

This is awesome! Thank you very much, that is exactly what I need.

[Gabriel439]

Fixed by #88

[lthms]

Thanks for the impressive reactivity. Do you plan to release a new version any time soon?

[Gabriel439]

Yes. I will probably publish a new version within about a week

[Gabriel439]

This is now up on Hackage as dhall-1.5.0