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Today's Topics:
1. Re: Opaque (to me) compilation error (Kim-Ee Yeoh)
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Message: 1
Date: Sat, 14 Nov 2015 16:44:24 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Opaque (to me) compilation error
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<capy+zdsahjpcqbs2ncvmizwatiqnxpqsu+vpmzltcedu9mj...@mail.gmail.com>
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On Sat, Nov 14, 2015 at 10:52 AM, Kim-Ee Yeoh <[email protected]> wrote:
> What we're seeing is special handling for stage 3: reporting errors in a
> do-block. Sometimes it helps, sometimes it hinders. For instance, the
> derivation of this signature is confusing indeed:
>
> IO System.IO.Handle
> -> (System.IO.Handle -> IO Data.ByteString.ByteString)
> -> Data.ByteString.ByteString
>
To elaborate, the error message comes from the compiler choking on the
monadic bind (>>=) that reveals itself in the desugared code.
The (>>=) results from desugaring
file <- openBinaryFile filename ReadMode
into
openBinaryFile filename ReadMode >>= \file -> ...
The compiler knows the type signature of (>>=), which wants to see
IO Handle -> (Handle -> IO ByteString) -> IO ByteString
But the signature of the top-level function insists on
IO Handle -> (Handle -> IO ByteString) -> ByteString
This is what's the compiler's trying to say.
-- Kim-Ee
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