Why do I get an exception loop - haskell

I tried following expression in prelude:
let x = x in x
and I've got following exception
Exception: <<loop>>
Why is the expression recursive?

let bindings in Haskell are (mutually) recursive, meaning that you can refer to any of the defined variables/functions (things to the left of the = signs) in any of their definitions (the stuff to the right of the = sign). For the case where you have arguments (functions), this is pretty much always the intuitive expected behaviour.
let fact n = if n == 0 then 1 else n * fact (n - 1) in fact 5
In the above, you probably are not surprised that fact (n - 1) can be used in the definition of fact n. In your example, you are using x in its own definition.
When Haskell tries to evaluate let x = x in x, it keeps trying to expand x (into the RHS x) hence the loop.

Related

Why does x*x where x = 6 fail in Haskell?

I've been reading thinking functionally with Haskell to learn Haskell and I came across that. In the book it says that A where clause does not qualify an expression but the whole of the right hand side of a definition. I don't quite understand this means.
The book means that the following "expression" is not valid Haskell syntax:
x*x where x = 6
You can see it is invalid by either entering it at the GHCi prompt:
> x*x where x=6
<interactive>:11:5: error: parse error on input ‘where’
or using it in a program where an expression is expected:
mybrokenprogram = (x * x where x = 6) -- gives a parse error
Contrast this with:
let x=6 in x*x
which works from the GHCi prompt:
> let x=6 in x*x
36
and works fine as part of the other example:
myawesomeprogram = (let x = 6 in x*x)
The book is explaining that a where clause applies to the entire right hand side of its associated defintion:
myfinewhereclause = x*x+x
where x = 6*6
Here, the where clause provides a definition for x that applies to the whole right-hand side x*x+x. Even if there was some alternative definition of x in scope, the where clause would redefine x for the whole RHS:
my_argument_x_is_ignored x = x*x+x where x = 6*6
Here, the where clause provides a definition of x that applies to every usage of x in the expression x*x+x. It doesn't apply only to the last use of x, and you can't use parenthesis to make it apply only to the last use of x:
wontwork x = x*x + (x where x=6*6) -- won't work: bad syntax
It's nothing deep. It's just the way the where syntax works.

Haskell | Are let expressions recalculated?

Lets say we have this function:
foo n = let comp n = n * n * n + 10
otherComp n = (comp n) + (comp n)
in (otherComp n) + (otherComp n)
How many times will comp n get actually executed? 1 or 4? Does Haskell "store" function results in the scope of let?
In GHCi, without optimization, four times.
> import Debug.Trace
> :{
| f x = let comp n = trace "A" n
| otherComp n = comp n + comp n
| in otherComp x + otherComp x
| :}
> f 10
A
A
A
A
40
With optimization, GHC might be able to inline the functions and optimize everything. However, in the general case, I would not count on GHC to optimize multiple calls into one. That would require memoizing and/or CSE (common subexpression elimination), which is not always an optimization, hence GHC is quite conservative about it.
As a thumb rule, when evaluating performance, expect that each (evaluated) call in the code corresponds to an actual call at runtime.
The above discussion applies to function bindings, only. For simple pattern bindings made of just a variable like
let x = g 20
in x + x
then g 20 will be computed once, bound to x, and then x + x will reuse the same value twice. With one proviso: that x gets assigned a monomorphic type.
If x gets assigned a polymorphic type with a typeclass constraint, then it acts as a function in disguise.
> let x = trace "A" (200 * 350)
> :t x
x :: Num a => a
> x + x
A
A
140000
Above, 200 * 350 has been recomputed twice, since it got a polymorphic type.
This mostly only happens in GHCi. In regular Haskell source files, GHC uses the Dreaded Monomorphism Restriction to provide x a monomorphic type, precisely to avoid recomputation of variables. If that can not be done, and duplicate computation is needed, GHC prefers to raise an error than silently cause recomputation. (In GHCi, the DMR is disabled to make more code work as it is, and recomputation happens, as seen above.)
Summing up: variable bindings let x = ... should be fine in source code, and work as expected without duplicating computation. If you want to be completely sure, annotate x with an explicit monomorphic type annotation.

What is a = a doing in haskell? [duplicate]

I was learning some new Haskell today, when I tried something in ghci.
It basically boiled down to this:
Prelude> let x = 6
Prelude> x
6
Prelude> let y = show x
Prelude> y
"6"
Prelude> let x = show x
Prelude> x
"\"\\\"\\\\\\\"\\\\\\\\\\\\\\\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\" --(repeats)
So can ghci not self-reference in assignment? I feel like it's akin to i = i++; in C, or trying to reference previous assignments of a let (not let*) in Scheme. Is there anyway to do this, or should I just use the easier let y = show x?
Definitions in Haskell are recursive by default. So the definition you made for x refers to the very same x, which is the reason for the very long string you are seeing, because x is defined to a String that is the result of calling show on itself, so you keep seeing the opening quotes for showing a string, and then those opening quotes escaped, and so on.
These kinds of definitions can be surprisingly useful. For example if you write:
let fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
you can define the Fibonacci sequence in terms of itself as an infinite list.
Then you can do something like take 10 fibs to just see the first 10 elements.
It's self referencing indeed! It's trying to solve the equation
x = show x
Since show returns a string Haskell knows that x begins like "\"" and that's enough to guess the second character, which is enough for the third which is enough for the fourth...
And so on.
Yes, you can do a not-selfreferencing assignment in ghci, although it's a bit cumbersome:
let x = 5
x <- return $ show x
lets are recursive, while monadic bindings are not.
Again: yes, Haskell very much can self-reference in assignment, otherwise this would just give an error instead of printing something undecipherable, wouldn't it?
What you can not do in Haskell, ever, is modify / re-assign the value of a variable. It's just totally out of question: if you've once defined x as some value, x will keep this value forever. An assumption baked into the very language, and used to great avail by the compiler for lots of optimisations etc..
Now you wonder why you can write let x = ... again at all, didn't I just say this is not possible? The thing is, what you're doing there is define a new variable that also happens to have the name x, but that doesn't change anything about the old variable with the same name. You might expect this
Prelude> let x = 6
Prelude> let p = print x
Prelude> let x = 7
Prelude> p
to yield 7, but actually it prints 6 because p is still referring to the old variable x, not to the new one that was defined only later.
Still confused? Perhaps less strange is something like
n :: Int
n = 7
f :: IO ()
f = print $ replicate n "ha"
... -- much later, you've forgotten there was a global `n` up there...
g :: String -> String
g = take n
where n = 37
It's quite reasonable that f will take 37 characters, not 7, while any call to f continues to repeat the string only 7 times. After all, it's "g, where n has that value", but nothing else outside. Now, let is just where written the other way around. In particular, a do notation or your GHCi prompt is actually syntactic sugar for something like this:
let x = 6
in ( print x >> ( let y = show x
in ( print y >> ( let x = show x
in ( print x )
)
)
)
Each paren encloses a scope. Variables defined in outer scopes can be used, but local ones are preferred if found. So let x = show x in ( print x ) can be considered completely on its own, the original x = 6 is shadowed out of scope here. Therefore, the only way the definition can work is refer recursively to itself.

Haskell - How do filter and head work in combination with where?

Here's the code.
largestDivisible :: (Integral a) => a
largestDivisible = head (filter p [100000,99999..])
where p x = x `mod` 3829 == 0
I am little bit confused. What is p in this case? Also, I do not understand the where expression in this particular example, because we got two expressions with p and x on the left side and we have one alignment, which is actually a boolean.
I would appreciate, if someone could explain me the above code.
p is a function, which accepts an argument x and returns True only if x is divisible by 3829. You can use where to define local functions just like you define local "values", using the same f x = y syntax you use to define top-level functions.

ghci self-referencing assignment

I was learning some new Haskell today, when I tried something in ghci.
It basically boiled down to this:
Prelude> let x = 6
Prelude> x
6
Prelude> let y = show x
Prelude> y
"6"
Prelude> let x = show x
Prelude> x
"\"\\\"\\\\\\\"\\\\\\\\\\\\\\\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\" --(repeats)
So can ghci not self-reference in assignment? I feel like it's akin to i = i++; in C, or trying to reference previous assignments of a let (not let*) in Scheme. Is there anyway to do this, or should I just use the easier let y = show x?
Definitions in Haskell are recursive by default. So the definition you made for x refers to the very same x, which is the reason for the very long string you are seeing, because x is defined to a String that is the result of calling show on itself, so you keep seeing the opening quotes for showing a string, and then those opening quotes escaped, and so on.
These kinds of definitions can be surprisingly useful. For example if you write:
let fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
you can define the Fibonacci sequence in terms of itself as an infinite list.
Then you can do something like take 10 fibs to just see the first 10 elements.
It's self referencing indeed! It's trying to solve the equation
x = show x
Since show returns a string Haskell knows that x begins like "\"" and that's enough to guess the second character, which is enough for the third which is enough for the fourth...
And so on.
Yes, you can do a not-selfreferencing assignment in ghci, although it's a bit cumbersome:
let x = 5
x <- return $ show x
lets are recursive, while monadic bindings are not.
Again: yes, Haskell very much can self-reference in assignment, otherwise this would just give an error instead of printing something undecipherable, wouldn't it?
What you can not do in Haskell, ever, is modify / re-assign the value of a variable. It's just totally out of question: if you've once defined x as some value, x will keep this value forever. An assumption baked into the very language, and used to great avail by the compiler for lots of optimisations etc..
Now you wonder why you can write let x = ... again at all, didn't I just say this is not possible? The thing is, what you're doing there is define a new variable that also happens to have the name x, but that doesn't change anything about the old variable with the same name. You might expect this
Prelude> let x = 6
Prelude> let p = print x
Prelude> let x = 7
Prelude> p
to yield 7, but actually it prints 6 because p is still referring to the old variable x, not to the new one that was defined only later.
Still confused? Perhaps less strange is something like
n :: Int
n = 7
f :: IO ()
f = print $ replicate n "ha"
... -- much later, you've forgotten there was a global `n` up there...
g :: String -> String
g = take n
where n = 37
It's quite reasonable that f will take 37 characters, not 7, while any call to f continues to repeat the string only 7 times. After all, it's "g, where n has that value", but nothing else outside. Now, let is just where written the other way around. In particular, a do notation or your GHCi prompt is actually syntactic sugar for something like this:
let x = 6
in ( print x >> ( let y = show x
in ( print y >> ( let x = show x
in ( print x )
)
)
)
Each paren encloses a scope. Variables defined in outer scopes can be used, but local ones are preferred if found. So let x = show x in ( print x ) can be considered completely on its own, the original x = 6 is shadowed out of scope here. Therefore, the only way the definition can work is refer recursively to itself.

Resources