Haskell: 漂亮的打印中缀和前缀

Question

我有一个类型来表示 haskell 类型：

data Type 
    = TApp Type Type
    | TVar Name
    | TLit Name
infixl 8 `TApp`

-- a -> b
aToB = TLit "Fun" `TApp` TVar "a" `TApp` TVar "b"

-- Maybe (IO Int)
maybeIOInt = TLit "Maybe" `TApp` (TLit "IO" `TApp` TLit "Int")

我想像haskell那样打印它，即作为符号的文字打印中缀，而其他文字打印前缀。必要时还应添加括号：

show aToB = "a -> b"
show maybeIOInt = "Maybe (IO Int)"

show ast = ???

我该如何实现？

Answer 1

执行此操作的通常方法是将优先级变量连接到您的打印函数。此外，您几乎总是应该更喜欢漂亮的打印库，而不是仅仅使用原始字符串（出于性能原因和易用性）。 GHC 附带 pretty and I also recommend the newer prettyprinter.

使用前者（并假设 type Name = String）：

import Text.PrettyPrint.HughesPJ

prettyType :: Type -> Doc
prettyType = go 0
  where
    go :: Int -> Type -> Doc
    go _ (TVar x) = text x
    go _ (TLit n) = text n
    go n (TLit "Fun" `TApp` l `TApp` r) = maybeParens (n > 0) (go 1 l <+> text "->" <+> go 0 r)
    go n (l `TApp` r) = maybeParens (n > 1) (go 1 l <+> go 2 r)

Haskell: 漂亮的打印中缀和前缀

Haskell: Pretty print infix and prefix

haskell

types

pretty-print

abstract-syntax-tree