You need to put a `forall a.` in front of the `Ord a` constraint. To quote the manual on the language extension[1]
Enable lexical scoping of type variables explicitly introduced with forall .
If it helps, the requirement of forall to be able to refer to the variable
was non-obvious to me the first time I tried to use the extension.
[1]
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts...
On Tue, Oct 9, 2018 at 11:45 AM Dennis Raddle
In the following snippet from a program in progress (designed to compute percentile rank for arbitrary lists of values) , I was hoping to declare types of functions within the main function just as a way of helping myself catch type errors. I'm getting the error "Can't match 'a' with 'a1'.... where 'a' is rigid type variable... etc. etc." on the line indicated in the comment below. The usual error I get when I try to do this without ScopedTypeVariables. So, I thought that ScopedTypeVariables was supposed to allow this kind of usage. What am I doing wrong?
{-# LANGUAGE ScopedTypeVariables #-}
import qualified Data.Map as M import qualified Data.List as L import Data.Map(Map) import Data.Function
-- <percent at or below> <percent below> data PercentileData = PercentileData Double Double
-- new attempt, October 2018: using new PercentileData construct to -- represent percentile in both ways. (at/below, or below) computePercentile :: Ord a => Map a Double -> Map a PercentileData computePercentile dataIn = error "foo" where pairs :: [(a,Double)] -- THIS IS THE LINE GETTING THE ERROR pairs = L.sortBy (compare `on` snd) $ M.toList dataIn
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