Calling function in haskell
I'm pretty new in Haskell programming. I want to call some functions and save the result in a variable but I don't know how. I read couple of chapters about haskell function in two different book but still don't understand how to do it.
import Data.Map (Map) import qualified Data.Map as M hiding (Map) newtype GenEnv elt = Env (Map Id elt) newEnv :: GenEnv elt -- initialise newEnv = Env M.empty newtype GenEnv elt = Env (Map Id elt) newEnv :: GenEnv elt -- initialise newEnv = Env M.empty getEnv :: GenEnv elt -> Id -> Maybe elt -- G(x) (key function) getEnv (Env env) var = M.lookup var env union :: GenEnv elt -> (Id,elt) -> GenEnv elt -- G[x:v] union (Env env) (key,elt) = Env (M.insert key elt env) -- foldr is faster than addToFM_list! unionL :: GenEnv elt -> [(Id,elt)] -> GenEnv elt -- list union unionL (Env env) pairs = Env $ foldr (\(k,e) g -> M.insert k e g) env pairs
What I'm asking here is NOT for somebody to do my work, asking how to call the functions because I don't understand their signature.
As others have mentioned, "variable" is perhaps not the right term. And in the same vein, "calling" a function is perhaps not the right term either. It is helpful, in my opinion, to think about this in terms of mathematical functions:
f(x) = x^2
given the above function, you don't "call" it with a value so much as give a name to the result of evaluating that function at a particular argument:
y = f(2)
It's the same in Haskell. Somewhere in your code you need to use the result of evaluating a function at a particular value. To do that, you can just use the application of that function to that value wherever you need it, or you can bind it to a name in a
let binding or a
So, to provide a simple example in Haskell, you can do something like this:
f :: Int -> Int f x = x^2 y :: Int y = f 2 g :: Int -> Int g x = let y = f 3 in y + 1 h :: Int -> Int h x = y + 1 where y = f 3
Here I have declared a function called
f which takes a single
Int value and returns a new
Int value, the square of the argument. Then I have declared an
Int value named
y to be the result of applying
2. The value
y is not a variable, but rather a binding. It will always be
Then I have declared two other functions,
h which are equivalent, showing local bindings of the results of applying
In your example, the types are perhaps complicating things a little bit.
Env is a constructor used to construct a value of the
Genenv type. So, to create a value that is a
Genenv type, you apply
Env to an appropriate argument. This is what
newEnv is doing.
Hopefully that's enough to get you started.
Haskell - Functions, As others have mentioned, "variable" is perhaps not the right term. And in the same vein, "calling" a function is perhaps not the right term either. Haskell is a functional language, so function calls and function definitions form a major part of any Haskell program. That's why the syntax for those two constructs is reduced to a bare minimum. Admittedly, this makes Haskell programs hard to read for newcomers.
Since I pretty much suck at explaining these things, I'd recomment reading this chapter
should cover everything you need to know to be able to call those functions.
In general in Haskell we apply a function to some arguments and bind the results to some value.
Calling function in haskell, Calling foreign functions. To bind a Haskell variable name and type to an external function, we introduce a new construct: foreign import. It defines the type of a Till now, we have discussed many types of Haskell functions and used different ways to call those functions. In this chapter, we will learn about some basic functions that can be easily used in Haskell without importing any special Type class. Most of these functions are a part of other higher order functions. Head Function
Haskell doesn't have variables, first of all. Generally we talk of binding an expression to an identifier, creating a thunk.
It has the form of
let val = unionEl g l in --expression using val
Calling foreign functions, Since building a PAP is a complicated business, instead we just behave as for an unknown function call, which will end up calling into the The Haskell report specifies that the main function always has type IO t, (for some type t which will be ignored) and hence never takes normal function arguments, so this is not the right approach. You are looking for the function getArgs (for which you have to import the module System.Environment .
In Haskell, function application is done through juxtaposition, but you can use parentheses first to get the feel for it, just don't use commas to separate parameters like you may be used to in other languages.
For instance, assuming Id is just a type synonym for Int, then the following would be an example GenEnv.
example :: GenEnv Char example = union newEnv (1, 'a')
which is the same as
example :: GenEnv Char example = union(newEnv (1, 'a'))
Say you have a list of the alphabet, associated with an integer value, called
alphabetKeys, you can use the handy little tool you defined to turn that into a GenEnv pretty easily, just by
example :: GenEnv Char example = union newEnv alphabetKeys
example :: GenEnv Char example = union(newEnv alphabetKeys)
would work just as well.
function calls · Wiki · Glasgow Haskell Compiler / GHC · GitLab, gcd 15 20 (which is a function call, incidentally) can be any integral type, but not a fractional type. There are five numeric types Higher Order Functions are a unique feature of Haskell where you can use a function as an input or output argument. Although it is a virtual concept, but in real-world programs, every function that we define in Haskell use higher-order mechanism to provide output.
Learn Haskell in 10 minutes, First, consider this definition of a function which adds its two arguments: Expressions that result in some kind of a run-time error, such as 1/0, also have this Haskell main function. Ask Question Asked 5 years, 8 months ago. Trouble calling a Haskell function from the main module. 2. Haskell frustration with function
A Gentle Introduction to Haskell: Functions, A foreign export declaration tells the compiler to expose a locally defined Haskell function to the outside world, i.e., wrap it up behind a calling interface that's Haskell has no prefix operators, with the exception of minus (-), which is both infix and prefix.] As another example, an important infix operator on functions is that for function composition: (.) :: (b->c) -> (a->b) -> (a->c) f . g = \ x -> f (g x) 3.2.1 Sections
Exposing Haskell functions, Using partial application (calling functions with too few parameters, if you will) is a neat way to create functions on the fly so we can pass them to another function The important concept to know in Haskell is guarded recursion (see tail recursion modulo cons), where any recursive calls occur within a data constructor (such as foldr, where the recursive call to foldr occurs as an argument to (:)).