Pretty printer for Template Haskell

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1

I use GHCi a lot, and want to look at the TH splices, and see how the results of my own TH, however, the default Show instance doesn’t do any indentaition. I know it is definitely possible to write the printer that would do what I want, with Data.Data, however is there already one defined?
E.G.
[d|instance Show (a -> b) where show _ = "Fun" |]
should give

[InstanceD 
    Nothing 
    [] 
    (AppT 
        (ConT GHC.Internal.Show.Show) 
        (AppT 
            (AppT 
                ArrowT 
                (VarT a_0))
            (VarT b_1))) 
    [FunD 
        GHC.Internal.Show.show 
        [Clause 
            [WildP]
            (NormalB 
                (LitE (StringL "Fun")))
            []]]]

I just want to get a printer like this, doesn’t need to be identitical.

2

TH comes with a pretty printer: pprint. It implements stuff from the pretty package so you can use the other typeclass methods too, but this is likely the function you want

3

This gives the desuagered haskell, not the AST. What it gives is essentially the same as the input I gave to the quasiquoter.

1 Like
4

Ah my bad! Maybe something like pretty-simple then, which just pretty prints the Show instance.

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