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Today's Topics:
1. My Continuation doesn't typecheck (martin)
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Message: 1
Date: Sat, 6 Aug 2016 10:03:17 +0200
From: martin <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] My Continuation doesn't typecheck
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
Hello all,
in order to gain some intuition about continuations, I tried the following:
-- two functions accepting a continuation
f1 :: Int -> (Integer->r) -> r
f1 a c = c $ fromIntegral (a+1)
f2 :: Integer -> (String -> r) -> r
f2 b c = c $ show b
-- combine the two functions into a single one
run1 :: Int -> (String -> r) -> r
run1 a = f1 a f2
-- *Main> run1 9 id
-- "10"
So far so good.
Then I tried to write a general combinator, which does not have f1 and f2
hardcoded:
combine a f g = f a g
-- This also works
-- *Main> combine 9 f1 f2 id
-- "10"
What confuses me is the the type of combine. I thought it should be
combine :: Int ->
(Int -> (Integer->r) -> r) -> -- f1
(Integer -> (String -> r) -> r) -> -- f2
((String -> r) -> r)
but that doesn't typecheck:
Couldn't match expected type ‘(String -> r) -> r’
with actual type ‘r’
Can you tell me where I am making a mistake?
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