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In some functional languages, partially applied functions and curried functions are two similar but distinct concepts.

Reading the book Learn You a Haskell for Great Good, the author appears to apply these interchangeably.

[Curried functions] Every function in Haskell officially only takes one parameter.

...

if we call a function with too few parameters, we get back a partially applied function, meaning a function that takes as many parameters as we left out.

My question is: Are the concepts of partial application and currying interchangeable in Haskell?

hawkeye
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  • The honest answer is short and it's yes. – hao Jun 20 '16 at 04:05
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    To me, "currying" has to do with function definition, whereas "partial application" has to do with function invocation. So they're not *exactly* interchangeable, but they're flip sides of the same coin, and neither is meaningful without the other. When you say that they are "similar but distinct" in some functional languages, what exactly are you referring to? – ruakh Jun 20 '16 at 04:08
  • It’s also interesting to note that, while the *type system* deals exclusively in curried unary functions, functions in GHC’s *generated code* take multiple arguments; closures are only allocated for partially applied functions. This is possible because the code has been typechecked under the curried interpretation, which is *isomorphic* to the uncurried interpretation—that is, it’s a type-preserving optimisation. – Jon Purdy Jun 20 '16 at 23:46

1 Answers1

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They are different but related concepts. Because function definitions are curried, when you partially apply a function (ie, call it with fewer arguments than it expects), you get back another function that expects fewer arguments.

The process of currying is replacing an N-ary function with N "layers" of unary functions so that you can partially apply it, turning

function(x,y) {return x+y;}

into

function(x) {return function(y) {return x+y;};}

The currying is part of defining the function, and partial application is what you do with a function when you call it.

amalloy
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