Haskell: The Craft of Functional Programming

Simon Thompson

(c) Addison-Wesley, 1999.

Chapter 11

> module Chapter11 where

Program development

^^^^^^^^^^^^^^^^^^^

Development in practice

^^^^^^^^^^^^^^^^^^^^^^^

Defining the .. notation (not executable code).

[m .. n]

| m>n = []

| otherwise = m : [m+1 .. n]

[1 .. n]

| 1>n = []

| otherwise = [1 .. n-1] ++ [n]

A simple palindrome check.

> simplePalCheck :: String -> Bool

> simplePalCheck st = (reverse st == st)

The full check

> palCheck = simplePalCheck . clean

where the clean function combines mapping (capitals to smalls) and

filtering (removing punctuation)

> clean :: String -> String

> clean = map toSmall . filter notPunct

> toSmall = toSmall -- dummy definition

> notPunct = notPunct -- dummy definition

Auxiliary functions

^^^^^^^^^^^^^^^^^^^

When is one string a subsequence of another?

> subseq :: String -> String -> Bool

> subseq [] _ = True

> subseq (_:_) [] = False

> subseq (x:xs) (y:ys)

> = subseq (x:xs) ys || frontseq (x:xs) (y:ys)

When is one strong a subsequece of another, starting at the front?

> frontseq :: String -> String -> Bool

> frontseq [] _ = True

> frontseq (_:_) [] = False

> frontseq (x:xs) (y:ys)

> = (x==y) && frontseq xs ys