Data declaration order matters to compiler.

{-# LANGUAGE GADTs #-}
{-# LANGUAGE TemplateHaskell #-}

import Control.Lens.TH (makeLenses)

data Handle h where
    PlayerHandle :: Int -> Handle Player

    -- GHC 7.8.3 complains that NonPlayer is not in scope
    NonPlayerHandle :: Int -> Handle NonPlayer

data Player = Player

-- Deferring this until after NonPlayer is defined, resolves the compile error.
-- Commenting this out also works.
makeLenses ''Player

data NonPlayer

(The exact version of 'lens' I have tried this with is 'lens-4.7' if that matters.)

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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changed weight to 5

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closed

This is (arguably unfortunately) the expected behavior. See the discussion of "declaration groups" in Section 7.17.1 of the User's Guide.

In short, splices delimit groups of declarations. Declarations can only be recursive within their group. The usual solution here would be to move the makeLenses splice after the data NonPlayer definition.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedInvalid

added Pnormal label