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Questions tagged [pattern-guards]

In function definitions by clauses with pattern matching, pattern guards allow for additional pattern matching inside guards, to determine which clause is chosen for execution. For Boolean guards use [guard-clause] tag.

In function definitions by clauses with pattern matching, pattern guards allow for additional pattern matching inside guards, to determine which clause is chosen for execution. For Boolean guards see .

References

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Erlang/Elixir guards and arity

Is there a way to see a function's guards without seeing the source code? Given an example function (in Elixir): def divide(x, y) when y != 0 do x / y end How would one figure out that there is a guard on divide/2 without access to the source…
Jason G
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How to patten match on a field of a state monad?

Is it possible to write write the function a using pattens matching/guards? {-# LANGUAGE PatternGuards #-} import Control.Monad.State.Strict(State, gets, runStateT) data MyState = MyState { counter :: Int } deriving (Show) a :: State…
Bilal Syed Hussain
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Fuzzy Pattern Matching Dispatch in Haskell

I'm writing some parse code over a tree. (Namely an Stanford nlp dependency tree) Basically I have a function like this: m :: DepTree -> Logic m (w, [E "nsubj" nsubj, E "dobj" dobj]) = ... m (w, [E "nsubj" nsubj, E "prep" prep]) = ... m (w, [E…
Thomas Ahle
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Haskell: where bindings pattern match

At the moment I'm trying to learn Haskell with the online tutorial Learn you a Haskell. In the chapter "Syntax in Functions" the author wrote "You can also use where bindings to pattern match!". After that there's a part of a code example, but I…
Daniel
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Suggested ScopedTypeVariables In a pattern type-signature

I Started writing Haskell code . I tried to write a fibonacci function using Guards - fibo :: (Num z, Ord z) => z -> z fibo d | d <= 0 = 0 | d == 1 = 1 | otherwise = fibo (d-1) + fibo (d-2) I got this error :- Illegal type…
New_coder
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Haskell: Pattern Syntax in expression context:_

I a fresh man in Haskell. Here is my program : maybe_divide :: Maybe Integer -> Maybe Integer -> Maybe Integer maybe_divide a b = case (a, b) of (Just a, Just b) | (Just a, Just b) | (Nothing, _) -> Nothing | (_, Just 0) ->…
akikaaa
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How do I specify that I want something to be present in Haskell

I have a guard and the condition is that lookup x list == something i.e. x is in the list. I tried: | lookup x list == _ = my code here But when loading the function I get a "pattern syntax in expression context" error?
James
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Haskell Says My Guard Has A Parse Error

So I've been playing with Haskell the past couple of days, and I decided I'd make a basic definition of the Fibonacci sequence. So I wrote this code: main = do fib :: (Integral a) => Int -> Int fib x | x == 0 = 0 | x == 1 =…
ThePhillipParadox
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why not a case with predicate guards in addition to pattern guards?

why not a case with predicate guards in addition to pattern guards? {-# LANGUAGE MultiWayIf, LambdaCase #-} module Main where import System.Info (os) import Control.Applicative ((<$>)) import Data.Char (toLower) import Data.List …
user1441998
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How to use matching on argument level, is it possible? Guards?

for example let When true d = d let foo = () |> When false So I've got side effect I don't like because it's error: MatchFailureException I know that I can have good side effect here: let foo = if false then () But warning Incomplete pattern…
cnd
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Haskell--Defining function with Guards

I am fairly new to Haskell and am working on an assignment simulating checkers currently. I am having a bit of difficulty determining the proper method of conditionally checking an expression and updating the values of a tuple. I have a function…
hashes4merkle
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Guards, Pattern Matching and different equations in Haskell

Getting back to my animals example: type Pig = String type Lion = String type Feed = [(Char,Char)] type Visitors = [(Char,Char)] type Costs = (Int,Int,Int) data AnimalHome = Farm Pig Pig Pig Feed | Zoo Lion Lion Lion Feed Visitors orders :: Char…
James
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