Capitalize a string

[Vapid]

I'm very new to functional programming. I'm trying to determine the best way to capitalize a string in Elm. I think I want to import string, char and then uncons, but I'm having trouble seeing how to pattern match against the returned monad to capitalize the first character and then glue the head back to the tail.

Thanks.

[Sean Corfield]

On Jan 27, 2015, at 5:29 PM, Vapid <vapid...@gmail.com> wrote:
> I'm very new to functional programming. I'm trying to determine the best way to capitalize a string in Elm. I think I want to import string, char and then uncons, but I'm having trouble seeing how to pattern match against the returned monad to capitalize the first character and then glue the head back to the tail.

When you use String.uncons you get back a value that is a Maybe data type, specifically Maybe (Char, String), and that can have one of two values: Just (c, s) - just a (character, string) tuple - or Nothing - no value. You can use case to pattern match on that:

case String.uncons s of
Just (c, ss) -> -- do something with c and ss
Nothing -> -- s was empty string

The second case is simple: the result should be s (the empty string) because capitalize "" should be "".

The first case is more interesting: the result should be the upper case version of c followed by (cons'd onto) the remainder of the string, which is ss:

Just (c, ss) -> String.cons (Char.toUpper c) ss

Putting that all together - and adding the ability to capitalize each word in a sentence - we get:

import Text
import String
import Char
import List

capitalize : String -> String
capitalize s =
case String.uncons s of
Just (c,ss) -> String.cons (Char.toUpper c) ss
Nothing -> s

capitalizeAll : String -> String
capitalizeAll s =
String.join " " <| List.map capitalize <| String.words s
-- or: String.join " " (List.map capitalize (String.words s))

main =
Text.plainText (capitalizeAll "hello, world!")

Sean Corfield -- (904) 302-SEAN
An Architect's View -- http://corfield.org/

"Perfection is the enemy of the good."
-- Gustave Flaubert, French realist novelist (1821-1880)

[Hassan Hayat]

Sorry, I misunderstood your question initially. I mistook capitalizing for uppercasing.

This is the solution I came up with and I'll walk you through it:

import String (..)

import Char 

capitalize : String -> String

capitalize string = 

  case uncons string of

    Nothing -> ""

    Just (head, tail) -> 

      append (fromChar (Char.toUpper head)) tail

You're right on the idea. So, uncons gives you a Maybe (Char, String) where the char is the first letter and the String is the rest of the letters.

case is how you do pattern matching in Elm. So it reads, if uncons string is:

1) Nothing. If I get nothing from unconsing it must mean that I have the empty string. So, let's just return the empty string cuz there's nothing better to do.

2) Just (head, tail). The head in this case is the first letter, the one you want to capitalize. so we call Char.toUpper on it. But Char.toUpper returns a char, so we need to convert it to a string, so we call fromChar which converts a char to a string. Note: there's also a general purpose toString function but all that does is surround anything with double quotes, so if you use that you'll drag in the single quotes along for the ride and that's not what you want.

And then, finally, we append that uppercased letter onto the rest of the string.

[Hassan Hayat]

Alternatively, you can just use String.cons. Sorry, I missed that

import String (..)

import Char

capitalize : String -> String

capitalize string =

  case uncons string of

   Nothing -> ""

   Just (head, tail) ->

      cons (Char.toUpper head) tail

In this case, you just "cons" the uppercased character onto the tail.

[Max Goldstein]

We should probably have common string processing methods in a third party library somewhere.

[Vapid Babble]

Excellent. Thanks for the help and thorough explanation. 

-jmwise

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