for package:cabal-install-solver

for is traverse with its arguments flipped. For a version that ignores the results see for_.

for_ is traverse_ with its arguments flipped. For a version that doesn't ignore the results see for. This is forM_ generalised to Applicative actions. for_ is just like forM_, but generalised to Applicative actions.

Examples

Basic usage:

>>> for_ [1..4] print
1
2
3
4

A variant of deepseq that is useful in some circumstances:

force x = x `deepseq` x

force x fully evaluates x, and then returns it. Note that force x only performs evaluation when the value of force x itself is demanded, so essentially it turns shallow evaluation into deep evaluation. force can be conveniently used in combination with ViewPatterns:

{-# LANGUAGE BangPatterns, ViewPatterns #-}
import Control.DeepSeq

someFun :: ComplexData -> SomeResult
someFun (force -> !arg) = {- 'arg' will be fully evaluated -}

Another useful application is to combine force with evaluate in order to force deep evaluation relative to other IO operations:

import Control.Exception (evaluate)
import Control.DeepSeq

main = do
result <- evaluate $ force $ pureComputation
{- 'result' will be fully evaluated at this point -}
return ()

Finally, here's an exception safe variant of the readFile' example:

readFile' :: FilePath -> IO String
readFile' fn = bracket (openFile fn ReadMode) hClose $ \h ->
evaluate . force =<< hGetContents h

Transformation that tries to enforce the rule that manual flags can only be set by the user. If there are no constraints on a manual flag, this function prunes all but the default value. If there are constraints, then the flag is allowed to have the values specified by the constraints. Note that the type used for flag values doesn't need to be Bool. This function makes an exception for the case where there are multiple goals for a single package (with different qualifiers), and flag constraints for manual flag x only apply to some of those goals. In that case, we allow the unconstrained goals to use the default value for x OR any of the values in the constraints on x (even though the constraints don't apply), in order to allow the unconstrained goals to be linked to the constrained goals. See https://github.com/haskell/cabal/issues/4299. Removing the single instance restriction (SIR) would also fix #4299, so we may want to remove this exception and only let the user toggle manual flags if we remove the SIR. This function does not enforce any of the constraints, since that is done by enforcePackageConstraints.

Traversal that tries to establish various kinds of user constraints. Works by selectively disabling choices that have been ruled out by global user constraints.

Enforce ghc's single instance restriction From the solver's perspective, this means that for any package instance (that is, package name + package version) there can be at most one qualified goal resolving to that instance (there may be other goals _linking_ to that instance however).