1 Replace syntactic sugar by a functionFor processing conditions, the
if' :: Bool -> a -> a -> a if' True x _ = x if' False _ y = y
Unfortunately there is no such function in the Prelude.
2.1 AdvantagesThe advantages of the function
- types: classification, documentation
- higher order functions: combinators
For example, the Hoogle query
[Bool] -> [a] -> [a] -> [a]
2.2 Use casesEach of the following functions could be defined in terms of
That function is harder to explain in English, than by its implementation. :-)
zipIf :: [Bool] -> [a] -> [a] -> [a] zipIf = zipWith3 if'
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')
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
2.4 If-Then-Else vs. guardsSome people (any exact statistics?) prefer guards to
2.5 Is If-Then-Else so important?Counting
- 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 we arrive at Haskell two some day, (http://haskell.org/hawiki/HaskellTwo (Web Archive)) 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
- Light proposal, compatible with Haskell 98: Add to the Prelude, maybe with a different name.if'
- Full proposal, incompatible with Haskell 98 and Haskell': Additionally remove syntaxif-then-else
2.9 See also
3 ObjectionsHaskell is not intended to be a minimalistic language, but to be one that is easy to read.
If properly indented, like
if a then b else c
if a then b else c