CODEWITHSUNDEEP
haskellcharactertuples
Diana
Diana

Reputation: 1437

Recursion on a list of tuples

I want to make a program that

I have the following code:

lookUp :: Char -> [(Char, Char)] -> Char    
lookUp a [] = []
lookUp a [(x,y),(xs,ys)]
   | a == x = y : lookUp [(xs,ys)]
   | otherwise = x : y : lookUp [(xs,ys)]

When I compile it, I get a lot of mistakes:

Couldn't match expected type 'char' with actual type [t0]

In an equation for 'lookUp'

and so on...

Sorry, I'm relatively new to Haskell. I'm pretty sure I made a mistake when dealing recursively with the tuple ([(x,y),(xs,ys)]), but I don't know how to change it. Any ideas?

Upvotes: 1

Views: 1289

Answers (1)

ErikR
ErikR

Reputation: 52039

This modification of your code will type check:

-- lookUp :: we'll have GHC tell us the type signature    
lookUp a [] = []
lookUp a ((x,y):pairs)
   | a == x = y : lookUp a pairs
   | otherwise = x : y : lookUp a pairs

Some obvious mistakes:

  • When you recursively call lookUp you only called it with one argument (you forgot the a argument)
  • The pattern [(x,y),(xs,ys)] will only match a list of exactly two pairs of characters. The pattern ((x,y):pairs) matches a non-empty list of pairs. The first pair is deconstructed into characters x and y and the remaineder of the list is bound to pairs.

Use the :t command in ghci to have GHC tell you what the type signature is.

Now whether or not this is what you want is another question.

Upvotes: 1

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