[Haskell-beginners] Multiline code in GHCi

[Brandon Allbery]

On Fri, Aug 7, 2015 at 11:15 AM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> Prelude> :set +m
> Prelude> let main1 = do
> Prelude|             a <- readLn
> Prelude|             return a
> Prelude|
> Prelude>
> Prelude> main1
>
> <interactive>:75:1:
>     No instance for (Show (IO b0)) arising from a use of `print'
>

In an actual program, it would be able to apply defaulting rules and
monomorphize `b0` to Integer. The somewhat unusual ghci environment, in
particular how it tries to figure out whether to evaluate an expression and
`print` the result or instead run an IO action --- coupled with the
monomorphism restriction being disabled in the interactive context in ghci
7.8 and later --- is working against you here. As a result, you need to
specify a result type when invoking `main1`, which is what the error
message is telling you.

-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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