ndmitchell / ndmitchell/hlint

Interesting reduction not caught by hlint.

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Dominant language
Haskell
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Description

So I have this function:

-- "Dual" of map, takes an object and a list of functions
-- and applies the object to each function in the list.
pam :: a -> [a -> b] -> [b]
pam a (f:fs) = f a : pam a fs
pam a [] = []

I was able to reduce it to:

pam :: a -> [a -> b] -> [b]
pam a = map (\f -> f a)

Sadly hlint didn't catch this reduction(Although it's able to catch a whole bunch of other reductions in my code any other day!), maybe this might lighten the way for a whole set of new reductions? Thanks for the awesome tool!

Contributor guide

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First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running HLint on the issue's pam definition and checking existing reduction suggestions for similar higher-order patterns. Trace the implementation and tests for those suggestions; done means HLint reports the equivalent map (\f -> f a) reduction for this example.

Written by the indexing model from the issue text.

Assessment

Tech stack
haskell
Domain
tooling
Issue type
Feature
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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