[Haskell-beginners] Opaque (to me) compilation error

Kim-Ee Yeoh ky3 at atamo.com
Sat Nov 14 03:52:37 UTC 2015

On Sat, Nov 14, 2015 at 9:24 AM, Theodore Lief Gannon <tanuki at gmail.com>

> You're missing IO in the type declaration, which I believe means that do
> block is running in the Id monad -- by inference, Id ByteString.

There's no running in the Id monad. The compiler doesn't understand monads
the way we'd like it to. It's all very syntactic, namely:

1. the compiler desugars the do block into an expression
2. it typechecks the expression: no special understanding of the monad
typeclass involved
3. and if there are errors, it reports them

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

-- Kim-Ee
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