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If-then-else

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1 Replace syntactic sugar by a function

For processing conditions,

the
if-then-else
syntax was defined in Haskell98. However it could be simply replaced by the function
if'

with

if' :: Bool -> a -> a -> a
if' True  x _ = x
if' False _ y = y

Unfortunately there is no such function in the Prelude.

2 Advocacy

2.1 Advantages

The advantages of the function
if'
over the syntax
if-then-else

are the same like for all such alternatives. So let me repeat two important non-syntactic strengths of Haskell:

  • types: classification, documentation
  • higher order functions: combinators
If
if'
would be a regular function,

each language tool can process it without hassle. Haddock can generate documentation for it, a text editor can make suggestions for values to insert, Hoogle can retrieve that function.

For example, the Hoogle query

[Bool] -> [a] -> [a] -> [a]

may return

zipWith3 if'


2.2 Use cases

Each of the following functions could be defined in terms of
if'
.

Actually, they do not even need to be in Prelude because they can be constructed so easily.

That function is harder to explain in English, than by its implementation. :-)

zipIf :: [Bool] -> [a] -> [a] -> [a]
zipIf = zipWith3 if'
Infix version. This duplicates the ternary operator of C-like languages, and can be used:
(cond ? exp1 $ exp2)
. Additionally, Church booleans can be represented compactly by sectioning on (?), i.e.
(True?) = const; (False?) = flip const
infixr 1 ?
(?) :: Bool -> a -> a -> a
(?) = if'

From a list of expressions choose the one, whose condition is true. The first parameter is the default value. It is returned if no condition applies.

select :: a -> [(Bool, a)] -> a
select = foldr (uncurry if')

See Case.

Easy lifting into monads (MonadReader in this case)

ifF :: (a -> Bool) -> (a -> b) -> (a -> b) -> (a -> b)
ifF = liftM3 if'


2.3 Why add this function to Prelude?

Actually people could define
if'
in each module,

where they need it,

or import it from a
Utility
module,

that must be provided in each project. Both solutions are tedious and contradict to modularization and software re-usage.

The central question is, whether
if'
is an idiom,

that is so general that it should be in the Prelude, or not. I think it is, otherwise it wouldn't have get a special syntax.

2.4 If-Then-Else vs. guards

Some people (any exact statistics?) prefer guards to
if-then-else
.

This practice has its own drawbacks, see Syntactic sugar/Cons and Things to avoid.


2.5 Is If-Then-Else so important?

Counting
if-then-else
or
if'

in today's Haskell programs

isn't a good measure for the importance a
if'
function,

because

  • frequently guards are used instead of
    if-then-else
  • there is no standard function, and this let people stick to work-arounds.

2.6 What is so bad about the if-then-else sugar?

Since syntactic sugar introduces its own syntactic rules, it is hard to predict how it interferes with other syntactic constructs. This special syntax for instance led to conflicts with do notation. A syntactic extension to solve this problem is proposed for Haskell'. It is not known what conflicts this extension might cause in future.


2.7 Why breaking lots of old and unmaintained code?

Haskell without
if-then-else
syntax

makes Haskell more logical and consistent. There is no longer confusion to beginners like: "What is so special about if-then-else, that it needs a separate syntax? I though it could be simply replaced by a function. Maybe there is some subtlety that I'm not able to see right now." There is no longer confusion with the interference of

if-then-else
syntax with
do
notation. Removing
if-then-else
simplifies every language tool,

say compiler, text editor, analyzer and so on.

If we arrive at Haskell two some day, (http://haskell.org/hawiki/HaskellTwo) it will certainly be incompatible to former Haskell versions. This does not mean, that old code must be thrown away. There should be one tool, that converts Haskell 98 and Haskell' to Haskell-2. Having one tool for this purpose is better than blowing all language tools with legacy code.

Syntactic replacements like
if-then-else
syntax to
if'
function should be especially simple.


2.8 Summary

  • Light proposal, compatible with Haskell 98: Add
    if'
    to the Prelude, maybe with a different name.
  • Full proposal, incompatible with Haskell 98 and Haskell': Additionally remove
    if-then-else
    syntax


2.9 See also


3 Objections

Haskell is not intended to be a minimalistic language, but to be one that is easy to read.

if-then-else
resembles a phrase from English language.

It shows clearly which expression is returned on a fulfilled condition, and which one is returned for an unsatisfied condition. It is thus easier to read. The special syntax saves parentheses around its arguments. If properly indented, like

if a
  then b
  else c

or

if a
  then b else c
then there is no conflict with the
do
-notation.