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When I try to compile
main = putStrLn $ show x where 2 + x = 7 GHC complains
error: Variable not in scope: x | 1 | main = putStrLn $ show x | ^ So it seems that 2 + x = 7 is by itself syntactically valid, although it doesn't actually define x. But why is it so?
801
It's merely an infix synonym for fmap , so you can write e.g. Prelude> (*2) <$> [1.. 3] [2,4,6] Prelude> show <$> Just 11 Just "11" Like most infix functions, it is not built-in syntax, just a function definition. But functors are such a fundamental tool that <$> is found pretty much everywhere.
The ++ operator is the list concatenation operator which takes two lists as operands and "combine" them into a single list.
(->) is often called the "function arrow" or "function type constructor", and while it does have some special syntax, there's not that much special about it. It's essentially an infix type operator. Give it two types, and it gives you the type of functions between those types.
Haskell also incorporates polymorphic types---types that are universally quantified in some way over all types. Polymorphic type expressions essentially describe families of types. For example, (forall a)[a] is the family of types consisting of, for every type a, the type of lists of a.
It is valid because it defines + instead.
main = print (3 + 4) where -- silly redefinition of `+` follows 0 + y = y x + y = x * ((x-1) + y) Above, the Prelude (+) function is shadowed by a local binding. The result will be 24, not 7, consequently.
Turning on warnings should point out the dangerous shadowing.
<interactive>:11:6: warning: [-Wname-shadowing] This binding for ‘+’ shadows the existing binding If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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