Automatically add type signatures to top-level functions

Question

I was lazy and wrote a Haskell module (using the excellent EclipseFP IDE) without giving type signatures to my top-level functions.

EclipseFP uses HLint to automatically label every offending function, and I can fix each one with 4 mouse clicks. Effective, but tedious.

Is there a utility program that will scan a .hs file, and emit a modified version that adds type signatures to each top-level function?

Example:

./addTypeSignatures Foo.hs 

would read a file Foo.hs:

foo x = foo + a

and emit

foo :: Num a => a -> a
foo x = x + 1

Bonus points if the tool automatically edits Foo.hs in place and saves a backup Foo.bak.hs

Answer 1

Here's a variation of the above script, that uses ":browse" instead of ":type", per ehird's comment.

One major problem with this solution is that ":browse" displays fully qualified type names, whereas ":type" uses the imported (abbreviated) type names. This, if your module uses unqualified imported types (a common case), the output of this script will fail compilation.

That shortoming is fixable (using some parsing of imports), but this rabbit hole is getting deep.

#!/usr/bin/env perl

use warnings;
use strict;

sub trim {
   my $string = shift;
   $string =~ s/^\s+|\s+$//g;
   return $string;
}


my $sig=0;
my $file;

my %funcs_seen = ();
my %keywords = ();
for my $kw qw(type newtype data class) { $keywords{$kw} = 1;}

foreach $file (@ARGV) 
{
  if ($file =~ /\.lhs$/) 
  {
    print STDERR "$file: .lhs is not supported. Skipping.\n";
    next;
  }

  if ($file !~ /\.hs$/) 
  {
    print STDERR "$file is not a .hs file. Skipping.\n";
    next;
  }

  my $module = $file;
  $module =~ s/\.hs$//;

  my $browseInfo = `echo :browse | ghci $file`;
  if ($browseInfo =~ /Failed, modules loaded:/)
  {
   print STDERR "$browseInfo\n";
   print STDERR "$file is not valid Haskell source file. Skipping.\n";
   next;
  }

  my @browseLines = split("\n", $browseInfo);
  my $browseLine;
  my $func = undef;
  my %dict = ();
  for $browseLine  (@browseLines) { 
   chomp $browseLine;
   if ($browseLine =~ /::/) {
    my ($data, $type) = split ("::", $browseLine);
    $func = trim($data);
    $dict{$func} = $type;
    print STDERR "$func :: $type\n";
   } elsif ($func && $browseLine =~ /^  /) { # indent on continutation
    $dict{$func} .= " " . trim($browseLine);
    print STDERR "$func ... $browseLine\n";
   } else {
    $func = undef;
   }
  }



  my $backup = "$file.bak";
  my $new = "$module.New.hs";
  -e $backup and die "Backup $backup file exists. Refusing to overwrite. Quitting";
  open OLD, $file;
  open NEW, ">$new"; 

  print STDERR "Functions in $file:\n";
  my $block_comment = 0;
  while (<OLD>) 
  {
    my $original_line = $_;
    my $line = $_;
    my $skip = 0;
    $line =~ s/--.*//;
    if ($line =~ /{-/) { $block_comment = 1;} # start block comment
    $line =~ s/{-.*//;
    if ($block_comment and $line =~ /-}/) { $block_comment=0; $skip=1} # end block comment

    if ($line =~ /^ *$/) { $skip=1; } # comment/blank
    if ($block_comment) { $skip = 1};
    if (!$skip) 
    {
      if (/^(('|\w)+)( +(('|\w)+))* *=/ ) 
      { 
        my $object = $1;
        if ((! $keywords{$object}) and !($funcs_seen{$object})) 
        {
          $funcs_seen{$object} = 1;
          print STDERR "$object\n";
          my $type = $dict{$1};

          unless ($sig) 
          {
            if ($type) {
              print NEW "$1 :: $type\n";
              print STDERR "$1 :: $type\n";
            } else {
              print STDERR "no type for $1\n";
            }
          }
        }
      }

    $sig = /^(('|\w)+) *::/; 
    }
    print NEW $original_line;
  }
  close OLD;
  close NEW;

  my $ghciPostTest = `echo 1 | ghci $new`;
  if ($ghciPostTest !~ /Ok, modules loaded: /)
  {
   print $ghciPostTest;
   print STDERR "$new is not valid Haskell source file. Will not replace original (but you might find it useful)";
   next;
  } else {
    rename ($file, $backup) or die "Could not make backup of $file -> $backup";
    rename ($new, $file) or die "Could not make new file $new";
  }
}

Answer 2

There's haskell-mode for emacs that has a shortcut to insert type signature of a function: C-u, C-c, C-t. It is not automatic, you have to do it for each function. But if you have only one module, it will probably take you a few minutes to go through it.

Answer 3

This perl script does a hack job at it, making some assumptions about source file structure. (Such as: .hs file (not .lhs), signatures are on the line immediately preceding definitions, definitions are flush on the left margin, etc)

It tries to handle (skip over) comments, equation-style definitions (with repeated left-hand-sides), and types that generate multi-line output in ghci.

No doubt, many interesting valid cases are not handled properly. The script isn't close to respecting the actual syntax of Haskell.

It is incredibly slow, as it launches a ghci session for each function that needs a signature. It makes a backup file File.hs.bak, prints the functions it finds to stderr, as well as signatures for functions missing signatures, and writes the upgraded source code to File.hs. It uses an intermediate file File.hs.new, and has a few safety checks to avoid overwriting your content with garbage.

USE AT YOUR OWN RISK.

This script might format your hard drive, burn your house down, unsafePerformIO, and have other impure side effects. In fact, it probably will.

I feel so dirty.

Tested on Mac OS X 10.6 Snow Leopard with a couple of my own .hs source files.

#!/usr/bin/env perl

use warnings;
use strict;

my $sig=0;
my $file;

my %funcs_seen = ();
my %keywords = ();
for my $kw qw(type newtype data class) { $keywords{$kw} = 1;}

foreach $file (@ARGV) 
{
  if ($file =~ /\.lhs$/) 
  {
    print STDERR "$file: .lhs is not supported. Skipping.";
    next;
  }

  if ($file !~ /\.hs$/) 
  {
    print STDERR "$file is not a .hs file. Skipping.";
    next;
  }

  my $ghciPreTest = `echo 1 | ghci $file`;
  if ($ghciPreTest !~ /Ok, modules loaded: /)
  {
   print STDERR $ghciPreTest;
   print STDERR "$file is not valid Haskell source file. Skipping.";
   next;
  }

  my $module = $file;
  $module =~ s/\.hs$//;

  my $backup = "$file.bak";
  my $new = "$module.New.hs";
  -e $backup and die "Backup $backup file exists. Refusing to overwrite. Quitting";
  open OLD, $file;
  open NEW, ">$new"; 

  print STDERR "Functions in $file:\n";
  my $block_comment = 0;
  while (<OLD>) 
  {
    my $original_line = $_;
    my $line = $_;
    my $skip = 0;
    $line =~ s/--.*//;
    if ($line =~ /{-/) { $block_comment = 1;} # start block comment
    $line =~ s/{-.*//;
    if ($block_comment and $line =~ /-}/) { $block_comment=0; $skip=1} # end block comment

    if ($line =~ /^ *$/) { $skip=1; } # comment/blank
    if ($block_comment) { $skip = 1};
    if (!$skip) 
    {
      if (/^(('|\w)+)( +(('|\w)+))* *=/ ) 
      { 
        my $object = $1;
        if ((! $keywords{$object}) and !($funcs_seen{$object})) 
        {
          $funcs_seen{$object} = 1;
          print STDERR "$object\n";
          my $dec=`echo ":t $1" | ghci $file  | grep -A100 "^[^>]*$module>" | grep -v "Leaving GHCi\." | sed -e "s/^[^>]*$module> //"`;

          unless ($sig) 
          {
            print NEW $dec;
            print STDERR $dec;
          }
        }
      }

    $sig = /^(('|\w)+) *::/; 
    }
    print NEW $original_line;
  }
  close OLD;
  close NEW;

  my $ghciPostTest = `echo 1 | ghci $new`;
  if ($ghciPostTest !~ /Ok, modules loaded: /)
  {
   print $ghciPostTest;
   print STDERR "$new is not valid Haskell source file. Will not replace original (but you might find it useful)";
   next;
  } else {
    rename ($file, $backup) or die "Could not make backup of $file -> $backup";
    rename ($new, $file) or die "Could not make new file $new";
  }
}

Answer 4

Here's another hacky attempt based on parsing GHC -Wmissing-signatures warnings, so the script doesn't have to parse Haskell. It transforms the warnings into a sed script that does the insertions and prints its result to stdout, or modifies the file inplace if -i is given.

Requires a Stack project as configured below, but you can change the buildCmd.

Works on the few files I tried it on with GHC 8.2.2 and 8.4.3, but same warnings as in @misterbee's first answer apply :) Also, it will obviously break with older or newer GHCs if they produce differently formatted warnings (but for me, the more sophisticated tooling seem to break all the time too, so...).

#!/bin/zsh

set -eu
setopt rematchpcre

help="Usage: ${0:t} [-d] [-i | -ii] HASKELL_FILE

Options:
  -d   Debug
  -i   Edit target file inplace instead of printing to stdout
           (Warning: Trying to emulate this option by piping from 
            and to the same file probably won't work!)
  -ii  Like -i, but no backup
"


### CONFIG ###

buildCmd() {
    touch $inputFile
    stack build --force-dirty --ghc-options='-fno-diagnostics-show-caret -Wmissing-signatures'
}

# First group must be the filename, second group the line number
warningRegexL1='^(.*):([0-9]+):[0-9]+(-[0-9]+)?:.*-Wmissing-signatures'

# First group must be the possible same-line type signature (can be empty)
warningRegexL2='Top-level binding with no type signature:\s*(.*)'

# Assumption: The message is terminated by a blank line or an unindented line
messageEndRegex='^(\S|\s*$)'


### END OF CONFIG ###


zparseopts -D -E d=debug i+=inplace ii=inplaceNoBackup h=helpFlag

[[ -z $helpFlag ]] || { printf '%s' $help; exit 0 }

# Make -ii equivalent to -i -i
[[ -z $inplaceNoBackup ]] || inplace=(-i -i)

inputFile=${1:P} # :P takes the realpath

[[ -e $inputFile ]] || { echo "Input file does not exist: $inputFile" >&2; exit 2 }

topStderr=${${:-/dev/stderr}:P}

debugMessage()
{
    [[ -z $debug ]] || printf '[DBG] %s\n' "$*" > $topStderr
}

debugMessage "inputFile = $inputFile"

makeSedScript() 
{
    local line

    readline() {
        IFS= read -r line || return 1
        printf '[build] %s\n' $line >&2
    }

    while readline; do
        [[ $line =~ $warningRegexL1 ]] || { debugMessage "^ Line doesn't match warningRegexL1"; continue }
        file=${match[1]}
        lineNumber=${match[2]}

        [[ ${file:P} = $inputFile ]] || { debugMessage "^ Not our file: $file"; continue }

        # Begin sed insert command
        printf '%d i ' $lineNumber

        readline

        [[ $line =~ $warningRegexL2 ]] ||\
            { printf 'WARNING: Line after line matching warningRegexL1 did not match warningRegexL2:\n %s\n' $line >&2
              continue }

        inlineSig=${match[1]}

        debugMessage "^ OK, inlineSig = $inlineSig"

        printf '%s' $inlineSig

        readline


        if [[ ! ($line =~ $messageEndRegex) ]]; then

            [[ $line =~ '^(\s*)(.*)$' ]]

            indentation=${match[1]}

            [[ -z $inlineSig ]] || printf '\\n'

            printf ${match[2]}

            while readline && [[ ! ($line =~ $messageEndRegex) ]]; do
                printf '\\n%s' ${line#$indentation}
            done
        fi

        debugMessage "^ OK, Type signature ended above this line"

        # End sed insert command
        printf '\n'

    done
}

prepend() {
    while IFS= read -r line; do printf '%s%s\n' $1 $line; done
}

sedScript="$(buildCmd |& makeSedScript)"

if [[ -z $sedScript ]]; then
    echo "No type-signature warnings for the given input file were detected (try -d option to debug)" >&2
    exit 1
fi

printf "\nWill apply the following sed script:\n" >&2
printf '%s\n' $sedScript | prepend "[sed] " >&2

sedOptions=()

if [[ $#inplace -ge 1 ]]; then 
    sedOptions+=(--in-place)
    [[ $#inplace -ge 2 ]] || cp -p --backup=numbered $inputFile ${inputFile}.bak
fi


sed $sedOptions -f <(printf '%s\n' $sedScript) $inputFile

Answer 5

For the Atom Editor its possible to automatically insert the type signature per function with the package haskell-ghc-mod which provides:

 'ctrl-alt-T': 'haskell-ghc-mod:insert-type'

https://atom.io/packages/haskell-ghc-mod#keybindings