[Haskell-cafe] Fwd: [Haskell-beginners] Monad instances and type synonyms

Christopher Howard christopher.howard at frigidcode.com
Sun Apr 14 07:10:14 CEST 2013

I asked this question in Haskell-beginners, but I haven't heard anything
yet, so I'm forwarding to Cafe.

-------- Original Message --------
Subject: [Haskell-beginners] Monad instances and type synonyms
Date: Sat, 13 Apr 2013 17:03:57 -0800
From: Christopher Howard <christopher.howard at frigidcode.com>
Reply-To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners at haskell.org>
To: Haskell Beginners <beginners at haskell.org>

I am playing around with some trivial code (for learning purposes) I
wanted to take

-- SaleVariables a concrete type defined early

-- `Adjustment' represents adjustment in a price calculation
-- Allows functions of type (a -> Adjustment a) to be composed
-- with an appropriate composition function
type Adjustment a = SaleVariables -> a

And put it into

instance Monad Adjustment where

  (>>=) = ...
  return = ...

If I try this, I get

Type synonym `Adjustment' should have 1 argument, but has been given none
In the instance declaration for `Monad Adjustment'

But if I give an argument, then it doesn't compile either (it becomes a
"*" kind). And I didn't want to make the type with a regular "data"
declaration either, because then I have to give it a constructor, which
doesn't fit with what I want the type to do.


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