A shell script to switch which version of GHC the non-versioned names point to

ghc-version

#!/bin/zsh

set -e
setopt null_glob

case $# in
    1) true;;
    *)
        echo Usage: ghc-version VERSION
        ghc --version || true
        echo You might try one of these versions:
        for dir in $path
        do
            for i in "$dir"/ghc-*
            do
                if echo "$i" | grep 'ghc-[-0-9.]*$' >/dev/null
                then
                    basename "$i" | cut -b 5-
                fi
            done
        done | sort -uV
        exit 1
        ;;
esac

version="$1"

success=true
for i in ghc ghci ghc-pkg runghc haddock-ghc
do
    if ! whence $i-"$version"
    then
        echo No target for $i-"$version" found.
        success=false
    fi
done
$success || echo Exiting without changing anything.
$success

for i in ghc ghci ghc-pkg runghc
do
    sudo ln -sf =$i-"$version" =$i
done

sudo ln -sf =haddock-ghc-"$version" =haddock