Error messages mention unification (?) variables like $coxswainTau0

[mrkgnao]

getAge :: (Lacks a "age", Short (NumCols a)) => R I (a .& "age" .= Int) -> Int
getAge = (`dot` #age)

getName :: (Lacks a "name", Short (NumCols a)) => R I (a .& "name" .= Text) -> Text
getName = (`dot` #name)

info :: _
info x = (getAge x, getName x)

The error:

src/Main.hs:47:9: error:
    • Found type wildcard ‘_’
        standing for ‘R I
                        (($coxswainTau0 .& ("name" .= Text)) .& ("age" .= Int))
                      -> (Int, Text)’
      Where: ‘$coxswainTau0’ is an ambiguous type variable
      To use the inferred type, enable PartialTypeSignatures
    • In the type signature: info :: _
   |
47 | info :: _
   |         ^
src/Main.hs:48:1: error:
    No instance for (Lacks $coxswainTau0 "age")
      arising from a use of ‘getAge’
   |
48 | info x = (getAge x, getName x)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[nfrisby]

Oh dear, I absolutely hadn't thought about type wildcards at all. Thank you very much for the example. The variable name is off-putting, but not necessarily problematic. The ambiguity warning on the other hand is concerning. I'll see what I can do, about both. (And special thanks for being my first bug reporter!)

…

On Sat, Oct 21, 2017, 00:01 Soham Chowdhury ***@***.***> wrote: getAge :: (Lacks a "age", Short (NumCols a)) => R I (a .& "age" .= Int) -> Int getAge = (`dot` #age) getName :: (Lacks a "name", Short (NumCols a)) => R I (a .& "name" .= Text) -> Text getName = (`dot` #name) info :: _ info x = (getAge x, getName x) The error: src/Main.hs:47:9: error: • Found type wildcard ‘_’ standing for ‘R I (($coxswainTau0 .& ("name" .= Text)) .& ("age" .= Int)) -> (Int, Text)’ Where: ‘$coxswainTau0’ is an ambiguous type variable To use the inferred type, enable PartialTypeSignatures • In the type signature: info :: _ |47 | info :: _ | ^ src/Main.hs:48:1: error: No instance for (Lacks $coxswainTau0 "age") arising from a use of ‘getAge’ |48 | info x = (getAge x, getName x) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#11>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABDg09eHz04pUByioQTFcSsWI1VxpVh2ks5suZbKgaJpZM4QBde_> .

[mrkgnao]

You're welcome! I'm trying to use your code to implement an extensible-effects library of some sort; let's see if I find any other bugs.

[nfrisby]

That's wonderful! I was very much hoping it would be useful for that.

…

On Sun, Oct 22, 2017, 00:04 Soham Chowdhury ***@***.***> wrote: You're welcome! I'm trying to use your code to implement an extensible-effects library of some sort; let's see if I find any other bugs. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#11 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABDg06X1IarHi5bAKQVIcfGL__jm4dvKks5suukTgaJpZM4QBde_> .