[Haskell-cafe] Error message??

[Brandon Allbery]

Did you perhaps confuse Haskell with a similar language (perhaps
Agda)? :: is a type ascription, while it looks like you might have
meant list construction (:).

On Wed, Dec 15, 2021 at 1:38 PM Gregory Guthrie <guthrie at miu.edu> wrote:
>
> I don't understand the error here:
>
> -- ------------------------------------------
> --  Find permutation by index
> iPerm :: Int -> [a] -> (Int,Int)
> iPerm k list = nPerm k $ length list
>    where nPerm k n = divMod k (fact(n - 1))
>
> -- indexPerm :: Int -> [a] -> [a]
> indexPerm _ [] = []
> indexPerm g list = d :: indexPerm r (delete d list)     -- line 19 ***
>    where (q,r) = iPerm g list
>          d = list !! q
>
> Adding a type signature for indexPerm changes the error (below),
> but seems to be the same basic issue.
>
> I thought I might need:
>     indexPerm :: Eq a => Int -> [a] -> [a]
> to match the restriction on delete argument types, although I would have thought that would be inferred if needed,
>   but it gives the same error message.
> -----------------------------------------------------------------------
> Prelude> (compiling)
> [1 of 1] Compiling Main             ( syntaxtest.hs, interpreted )
>
> yntaxtest.hs:19:20: error:
>     * Couldn't match expected type `indexPerm r (delete1 d1 list1)'
>                   with actual type `a'
>         because type variables `indexPerm', `r', `delete1', `d1', `list1'
>         would escape their scope
>       These (rigid, skolem) type variables are bound by
>         an expression type signature:
>           forall (indexPerm :: * -> * -> *) r (delete1 :: * -> * -> *) d1 list1.
>           indexPerm r (delete1 d1 list1)
>         at syntaxtest.hs:19:25-51
>     * In the expression: d :: indexPerm r (delete d list)
>       In an equation for `indexPerm':
>           indexPerm g list
>             = d :: indexPerm r (delete d list)
>             where
>                 (q, r) = iPerm g list
>                 d = list !! q
>     * Relevant bindings include
>         d :: a (bound at syntaxtest.hs:21:10)
>         list :: [a] (bound at syntaxtest.hs:19:13)
>        indexPerm :: Int -> [a] -> [delete d list]
>           (bound at syntaxtest.hs:18:1)
>    |
> 19 | indexPerm g list = d :: indexPerm r (delete d list)
>
>    |                    ^
>
>    |
> 19 | indexPerm g list = d :: indexPerm r (delete d list)
>
>    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Failed, no modules loaded.
> -----------------------------------------------------------------------
> (error with the explicit type signature)
>
> syntaxtest.hs:19:20: error:
>     * Couldn't match expected type `indexPerm r (delete d list)'
>                   with actual type `a'
>       `a' is a rigid type variable bound by
>         the type signature for:
>           indexPerm :: forall a. Int -> [a] -> [a]
>         at syntaxtest.hs:17:1-30
>     * In the expression: d :: indexPerm r (delete d list)
>       In an equation for `indexPerm':
>           indexPerm g list
>             = d :: indexPerm r (delete d list)
>             where
>                 (q, r) = iPerm g list
>                 d = list !! q
>     * Relevant bindings include
>         d :: a (bound at syntaxtest.hs:21:10)
>         list :: [a] (bound at syntaxtest.hs:19:13)
>         indexPerm :: Int -> [a] -> [a] (bound at syntaxtest.hs:18:1)
>    |
> 19 | indexPerm g list = d :: indexPerm r (delete d list)
>
>    |                    ^
>
>    |
> 19 | indexPerm g list = d :: indexPerm r (delete d list)
>
>    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Failed, no modules loaded.
>
>
> Gregory Guthrie
>
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com