干净-附加到列表

Question

I am trying to append to a list in the Clean programming language .

This is my code:

import StdEnv

path0 = "urrd"
//path1 = "urdl"
//path2 = "uurrddll"
//path3 = "ururdrdldlul"

Mirror :: [Char] -> [Char]

Mirror [] = []
Mirror str = [(NegatePath last str) : (Mirror init str)]

NegatePath :: Char -> Char

NegatePath 'u' = 'd'
NegatePath 'd' = 'u'
NegatePath 'l' = 'r'
NegatePath 'r' = 'l'

Start = Mirror path0

This is the error I get:

Type error [Homokozoo.icl,13,Mirror]:"argument 1 of list constructor" cannot unify types: [Char] {#Char}

Answer 1

I didn't know about the Clean language before reading this question, so this could be way off, but I think I've identified the problem after spending some time scanning chapter 4 of the language report. The problem appears to be that your Mirror function is built to operate on a list of characters [Char] but you are passing it an unboxed array of characters {#Char} .

It might work if you declared

path0 = ['urrd']

so that the type of path0 is a list of characters rather than an unboxed array of characters. If that's not what you want, you may need to modify the Mirror function to work on {Char} .