[Haskell-cafe] Annoying problem when pattern matching negative integer literals and wildcards

[Brandon Allbery]

Maybe one of the numeric literal extensions will help. The root problem is
that, without parentheses or one of the extensions, this ends up defining
something different than you intended: (-) with `encryptionResult` as a
local binding.

On Thu, Mar 14, 2019 at 11:55 PM Vanessa McHale <vanessa.mchale at iohk.io>
wrote:

> I have the following program:
>
> module Bug ( encryptionResult ) where
>
> data EncryptionResult = HasEncryption
>                       | EncryptionUnknown
>
> encryptionResult :: Int -> EncryptionResult
> encryptionResult 1 = HasEncryption
> encryptionResult -1 = EncryptionUnknown
> encryptionResult _ = error "Internal error."
>
> When I try to compile it with GHC I get
>
> [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
>
> Bug.hs:9:1: error:
>     Multiple declarations of ‘encryptionResult’
>     Declared at: Bug.hs:7:1
>                  Bug.hs:9:1
>   |
> 9 | encryptionResult _ = error "Internal error."
>   | ^^^^^^^^^^^^^^^^
>
> I can replicate this in Hugs, viz.
>
> ERROR "Bug.hs":7 - "encryptionResult" multiply defined
>
> However, everything compiles fine when I write
>
> module Bug ( encryptionResult ) where
>
> data EncryptionResult = HasEncryption
>                       | EncryptionUnknown
>
> encryptionResult :: Int -> EncryptionResult
> encryptionResult 1 = HasEncryption
> encryptionResult -1 = EncryptionUnknown
>
> or
>
> module Bug ( encryptionResult ) where
>
> data EncryptionResult = HasEncryption
>                       | EncryptionUnknown
>
> encryptionResult :: Int -> EncryptionResult
> encryptionResult 1 = HasEncryption
> encryptionResult 0 = EncryptionUnknown
> encryptionResult _ = error "Internal error."
>
> Am I doing something obviously screwy? This seems like a pretty annoying
> feature on the language (to the point where I assumed it was a GHC bug
> until I got the same behavior with Hugs) and I can't figure out why it
> exists.
>
> Cheers,
> Vanessa McHale
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com
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