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

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 2

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";
  }
}