我正在尝试将Idris翻译成Cayenne的一个例子 – 一种依赖类型为
paper的语言.
这是我到目前为止
PrintfType : (List Char) -> Type
PrintfType Nil = String
PrintfType ('%' :: 'd' :: cs) = Int -> PrintfType cs
PrintfType ('%' :: 's' :: cs) = String -> PrintfType cs
PrintfType ('%' :: _ :: cs) = PrintfType cs
PrintfType ( _ :: cs) = PrintfType cs
printf : (fmt: List Char) -> PrintfType fmt
printf fmt = rec fmt "" where
rec : (f: List Char) -> String -> PrintfType f
rec Nil acc = acc
rec ('%' :: 'd' :: cs) acc = \i => rec cs (acc ++ (show i))
rec ('%' :: 's' :: cs) acc = \s => rec cs (acc ++ s)
rec ('%' :: _ :: cs) acc = rec cs acc -- this is line 49
rec ( c :: cs) acc = rec cs (acc ++ (pack [c]))
我使用列表字符而不是字符串作为格式参数,以便于模式匹配,因为我很快遇到了与字符串模式匹配的复杂性.
不幸的是,我收到一条错误消息,我无法理解：
Type checking ./sprintf.idr
sprintf.idr:49:Can't unify PrintfType (Prelude.List.:: '%' (Prelude.List.:: t cs)) with PrintfType cs
Specifically:
Can't convert PrintfType (Prelude.List.:: '%' (Prelude.List.:: t cs)) with PrintfType cs
如果我在PrintfType和printf中注释掉3个元素(带有’％’:: …的元素)的所有模式匹配,那么代码编译(但显然不会有任何有趣的事情).
如何修复我的代码,使printf“％s是％d”“答案”42作品？
看起来像idris中有一些
current limitations在定义模式重叠的函数(例如’％’::’d’与c :: cs重叠时,经过很多尝试,我终于找到了一个解决方法：
原文链接：https://www.f2er.com/javaschema/281776.htmldata Format = End | FInt Format | FString Format | FChar Char Format
fromList : List Char -> Format
fromList Nil = End
fromList ('%' :: 'd' :: cs) = FInt (fromList cs)
fromList ('%' :: 's' :: cs) = FString (fromList cs)
fromList (c :: cs) = FChar c (fromList cs)
PrintfType : Format -> Type
PrintfType End = String
PrintfType (FInt rest) = Int -> PrintfType rest
PrintfType (FString rest) = String -> PrintfType rest
PrintfType (FChar c rest) = PrintfType rest
printf : (fmt: String) -> PrintfType (fromList $unpack fmt)
printf fmt = printFormat (fromList $unpack fmt) where
printFormat : (fmt: Format) -> PrintfType fmt
printFormat fmt = rec fmt "" where
rec : (f: Format) -> String -> PrintfType f
rec End acc = acc
rec (FInt rest) acc = \i: Int => rec rest (acc ++ (show i))
rec (FString rest) acc = \s: String => rec rest (acc ++ s)
rec (FChar c rest) acc = rec rest (acc ++ (pack [c]))
格式是表示格式字符串的递归数据类型. FInt是一个int占位符,FString是字符串占位符,FChar是一个字符文字.使用格式我可以定义PrintfType并实现printFormat.从那里,我可以顺利地扩展到一个字符串,而不是列表字符或格式值.最终的结果是：
*sprintf> printf "the %s is %d" "answer" 42 "the answer is 42" : String
