I'm trying to understand this function (taken from here)
escape :: String -> String
escape =
let
escapeChar c =
case c of
'<' -> "<"
'>' -> ">"
_ -> [c]
in
concat . map escapeChar
My questions are:
escape is a function that takes a String. But it seems that in in the fuction definition it does not receive any argument. How does this work?escapeChar and c? How is that relationship established? Does c coming right after escapeChar have a meaning?Would it be easier if escapeChar were a top-level definition using pattern matching:
escape :: String -> String
escape = concatMap escapeChar
escapeChar :: Char -> String
escapeChar '<' = "<"
escapeChar '>' = ">"
escapeChar ch = [ch]
[ch] is a singleton list, it turns ch :: Char into a [ch] :: String.
In Haskell you can remove/add an argument from/to each side (eta conversion). escape is the eta reduced form of
escape :: String -> String
escape str = concatMap escapeChar str
Just like, if you wanted to define a synonym for (+) you have equivalent ways of writing it. I feel like the add = (+) is clearest, you are identifying the two functions. The arguments are the same on both sides so we don't specify them.
add :: Int -> Int -> Int
add = (+)
add a = (+) a
add a = (a +)
add a b = (+) a b
add a b = a + b
These are equivalent ways of writing escape:
escape = concat . map escapeChar
escape str = concat (map escapeChar str)
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