gistfile1.txt

NOTE: the types used below, e.g. Name or UnQual come from Language.Haskell.Exts

I am using generic-lens to do this kind of traversal:

putStrLn $ show $ toListOf (types @(Name SrcSpanInfo)) foo


When foo is this expression it works fine:

(UnQual (SrcSpanInfo {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32, srcInfoPoints = []}) 
    (Ident (SrcSpanInfo {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32, srcInfoPoints = []}) "DeclHead"))
        
When foo is this expression it does not compile: 

(TyCon (SrcSpanInfo {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32, srcInfoPoints = []}) 
    (UnQual (SrcSpanInfo {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32, srcInfoPoints = []}) 
    (Ident (SrcSpanInfo {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32, srcInfoPoints = []}) "DeclHead")))

The error message GHC gives is: 

    • | No instance for ‘Generic (GHC.Real.Ratio Integer)’
      |   arising from a generic traversal.
      |   Either derive the instance, or define a custom traversal using ‘HasTypesCustom’

    • In the first argument of ‘toListOf’, namely
        ‘(types @(Name SrcSpanInfo))’
      In the expression: toListOf (types @(Name SrcSpanInfo))
      In the second argument of ‘($)’, namely
        ‘toListOf (types @(Name SrcSpanInfo))
         $ (TyCon
              (SrcSpanInfo
                 {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32,
                  srcInfoPoints = []})
              (UnQual
                 (SrcSpanInfo
                    {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32,
                     srcInfoPoints = []})
                 (Ident
                    (SrcSpanInfo
                       {srcInfoSpan = SrcSpan "Syntax.hs" 268 24 268 32,
                        srcInfoPoints = []})
                    "DeclHead")))’
 
For your reference, the constructor used above, TyCon, is part of the algebraic data type "Type" defined in 
https://hackage.haskell.org/package/haskell-src-exts-1.21.0/docs/Language-Haskell-Exts-Syntax.html#t:Type
 
I noticed that the constructor  Splice  refers to Exp, so this may be the constructor that pulls in Ratio.

Since I can't get rid of Ratio, I am going to need to conjure a Generic instance for it. If anyone has 
guidance on that I would greatly appreciate it.