functional programming - Haskell function definition syntax -

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i'm doing lists concatenation in following ways (an example, using ghc):

myconcat :: [[a]] -> [a] myconcat xs = foldr (++) [] xs myconcat    = foldr (++) []

can explain me please why , how above definitions work , 1 not:

myconcat xs = foldr (++) []

is last line of code intentionally not allowed (for reason constructs might become confusing, useless, etc) or deeper, maybe related currying...

i hope can shed light on this, puzzles me :/

later edit: besides explanations given below, i've found source of information on matter section "partial function application , currying" chap. 4 "functional programming" book "real world haskell". book available freely online.

lets review different versions:

myconcat xs = foldr (++) [] xs

this usual way, providing argument, consumed foldr. type [[a]] -> [a], because have argument of type [[a]] on left side, yields [a] when fed right side. 

myconcat = foldr (++) []

here foldr partially applied, return function can take additional argument, list of lists. right side need, not "syntactical suger", way express same thing first version. type again [[a]] -> [a]: have nothing on left side, give function of signature @ right side.

myconcat xs = foldr (++) []

here foldr partially applied well, , return function can take argument before, our definition has additional argument xs, isn't used on right side. compiler doesn't "know" this argument want have applied right side. type t -> [[a]] -> [a]. why?

assume have square function:

sqr :: int -> int  sqr x = x*x

what doing same providing addition, unused argument:

sqr:: int -> t -> int  sqr x y = x*x

the function still "works", e.g. sqr 3 "bla" yields 9, type signature off, , unused argument is... erm, unused. unused argument has no fixed type, can virtually "anything", doesn't matter. gets type variable (t) in signature.


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