make For Data Using Makefiles

Mr. Mather recently spotted the Makefile we've been using for the updated version of Toner and asked for a walk-through.

Like many other people who wrangle data for a living, I've been in on-and-off pursuit of a make-like approach to processing and transforming data. Specifically, one that allows me to idempotently bootstrap and process data without starting from zero each time. Also, one that makes it easy enough to script experiments pro-actively rather than putting replicability off.

Common Idioms

We use a number of sh / bash idioms that are helpful to recognize as such.

Use of Exit Codes

When a process exits, it does so with a numeric code that indicates success or failure (in the latter case, often with a value that can be used to determine why). 0 is considered success; anything else is a failure of some sort. These codes aren't directly visible, but make uses them to determine whether a target was successful (and whether execution should continue). $? can be used to surface the code in a shell context:

$ echo hi; echo $?
hi
0

There are a few cases where we want to shield make from failures (to prevent execution from stopping). For example:

carto -l $< > $@ || (rm -f $@; false)

This runs carto; if it fails, it deletes the output ($@). However, if there was no output, rm will return successfully, but the task actually failed. To work-around that, we execute rm in a subshell (using parentheses) and explicitly return false (false) so that make ceases execution.

|| and &&

|| and && are used in conjunction with exit codes to conditionally execute subsequent parts of a composite command. || is used when you want to execute a command only if the first returns an error (non-zero exit code); && when you want to execute a command only if the first was successful.

> /dev/null 2>&1

POSIX processes are provided 3 file descriptions (handles to files or file-like things) by default. stdin (content from a file or other source like a keyboard) is file descriptor 0, stdout is 1, and stderr is 2. Thus, echo hi > /dev/null 2>&1 says to redirect stdout (the default output) to /dev/null (into the abyss) and then stderr (2) to the same place as stdout (as a reference: &1). In this context, we use this to squelch output, since we generally just care about the exit code (either directly, or after greping through it or counting lines (wc -l)).

psql Checks

At various points, we want to short-circuit tasks if a resource already exists. In a traditional Makefile, these resources would be files and that short-circuiting is built-in. However, since we're since working with database tables (etc.) and there are no file equivalents, we need to implement equivalent functionality.

We use psql -c for practically all existence checks, as it allows us to construct a SQL command and use it to query PostgreSQL. Unfortunately, different checks result in varying output and exit codes.

Looking for Relations

\d <relation> will check for the existence of a "relation" (which could be a table, view, etc.). It displays the name, type, and owner of the relation and exits with 0 only if it matched something.

Looking for Extensions

\dx <extension> will check for extensions matching the provided name. If present, it will display various information about it. Unfortunately, it exits with 0 regardless of whether anything has been found or not, so we need to grep the output for the presence of the name.

Looking for Functions

\df <name> will check for functions matching the provided name. When a function has multiple signatures, it will be displayed multiple times. Like \dx, it exits with 0 no matter what, so we need to grep (case-insensitively) for the function we're looking for.

Implicit make Variables

make includes many implicit variables, most of which are intended for use in an environment where source files are being compiled and linked into binaries. However, there are still a few we make use of:

• $@ - the name of the target (the thing before the :).
• $< - the name of the first prerequisite. Convenient.
• $^ - all prerequisites, space-delimited. Can be combined with $(word <n>,$^) to select the nth one.

make Functions

Since many of the targets we're working with are synthetic, we need to extract relevant components of their names. Database-related functionality is grouped under the db/ "path", so we primarily use subst to remove irrelevant components. We also use word to refer to components within space-delimited values.

make Patterns

There are many resources that follow patterns when we work with data. Natural Earth's filenames and source URLs are a good example of this. In keeping with the DRY principle ("don't repeat yourself"), we fold these into a smaller number of targets using make patterns. These are strings that include % anywhere text may vary.

The convenient bit is that prerequisites can also use the % syntax and the value of the pattern in the target name will be substituted. Thus, %: %.mml will convert toner to toner: toner.mml and gives us the behavior we're looking for.

Annotated Makefile

Without further ado, here's an annotated snapshot of the Makefile-driven approach we've been using. Suggestions are most welcome, especially if they simplify or clarify.

SHELL := /bin/bash

Use bash for sub-shells, allowing use of bash-specific functionality.

PATH := $(PATH):node_modules/.bin

Add npm-installed binaries to the PATH.

define EXPAND_EXPORTS
export $(word 1, $(subst =, , $(1))) := $(word 2, $(subst =, , $(1)))
endef

Define a macro that expands (splits on =) and exports (makes available to sub-shells) key-value arguments, e.g. DATABASE\_URL=postgres:///db.

# wrap Makefile body with a check for pgexplode
ifeq ($(shell test -f node_modules/.bin/pgexplode; echo $$?), 0)

Check whether pgexplode has been installed (it's effectively a prerequisite for the entire Makefile); test doesn't output anything, so the return code ($?) needs to be echo'd for comparison with 0 (and escaped with an extra $ to be passed through to the shell call).

# load .env
$(foreach a,$(shell cat .env 2> /dev/null),$(eval $(call EXPAND_EXPORTS,$(a))))

Read .env (squelching error messages if one doesn't exist) and pass each environment pair to EXPAND\_EXPORTS to make it available to commands in targets.

# expand PG* environment vars
$(foreach a,$(shell set -a && source .env 2> /dev/null; node_modules/.bin/pgexplode),$(eval $(call EXPAND_EXPORTS,$(a))))

Use pgexplode to expand DATABASE\_URL into libpq-compatible environment variables. This will read from the environment ($DATABASE\_URL) if one isn't present in .env (or .env doesn't exist).

define create_relation
@psql -c "\d $(subst db/,,$@)" > /dev/null 2>&1 || \
        psql -v ON_ERROR_STOP=1 -qX1f sql/$(subst db/,,$@).sql
endef

define create_extension
@psql -c "\dx $(subst db/,,$@)" | grep $(subst db/,,$@) > /dev/null 2>&1 || \
        psql -v ON_ERROR_STOP=1 -qX1c "CREATE EXTENSION $(subst db/,,$@)"
endef

Macro definitions that will strip db/ from targets' $@ (target name) and use it as the name of a SQL file or PostgreSQL extension ($(subst db/,,$@)). The first command (psql -c "\d $(subst db/,,$@)" > /dev/null 2>&1) checks for the presence of a relation in the database specified by DATABASE\_URL and only evaluates SQL commands if it fails. \d <name> checks for the presence of a relation of any kind, \dx for loaded extensions, etc.

ON\_ERROR\_STOP=1 is used to abort as soon as an error occurs, -q is "quiet", -X ignores any psqlrc files, -1 runs the command in a single transaction, -f provides a file containing commands, and -c tells it to use the provided command.

Commands are prefixed with @ to prevent make from printing them (since they're unnecessarily complicated due to the need to check for the existence of non-file resources).

define register_function_target
.PHONY: db/functions/$(strip $(1))

db/functions/$(strip $(1)): db
        @psql -c "\df $(1)" | grep -i $(1) > /dev/null 2>&1 || \
                psql -v ON_ERROR_STOP=1 -qX1f sql/functions/$(1).sql
endef

$(foreach fn,$(shell ls sql/functions/ 2> /dev/null | sed 's/\..*//'),$(eval $(call register_function_target,$(fn))))

Generate targets for each file in sql/functions, callable as db/function/<name> and depending on the db target (keep reading).

# Import PBF ($2) as $1
define import
.PHONY: db/osm-$(strip $(word 1, $(subst :, ,$(1)))) db/$(strip $(word 1, $(subst :, ,$(1))))

db/$(strip $(word 1, $(subst :, ,$(1)))): db/osm-$(strip $(word 1, $(subst :, ,$(1)))) db/shared

db/osm-$(strip $(word 1, $(subst :, ,$(1)))): db/postgis db/hstore $(strip $(word 2, $(subst :, ,$(1))))
        @psql -c "\d osm_roads" > /dev/null 2>&1 || \
        imposm3 import \
                --cachedir cache \
                -mapping=imposm3_mapping.json \
                -read $(strip $(word 2, $(subst :, ,$(1)))) \
                -connection="$${DATABASE_URL}" \
                -write \
                -deployproduction \
                -overwritecache
endef

Macro definition for importing OSM extracts. When called, it generates targets like db/<place> (which depends on both the import and db/shared (see below)) and db/osm-<place>, which does the actual import. osm\_roads is assumed to be created by imposm3 in this case. $${DATABASE\_URL} is escaped because this is a macro (so it's evaluated at runtime) and uses braces to use the environment variable.

Now begins what looks like a more conventional Makefile.

# default target
default: toner

# symlink into TileMill's project folder (if TM is installed)
link:
        @test -e ${HOME}/Documents/MapBox/project && \
                test -e ${HOME}/Documents/MapBox/project/toner || \
                ln -sf "`pwd`" ${HOME}/Documents/MapBox/project/toner

# clean up derivative files
clean:
        @rm -f *.mml *.xml

# create a default .env file with a sensible(?) default
.env:
        @echo DATABASE_URL=postgres:///toner > $@

%: %.mml
        @cp $< project.mml

A pattern, which obviates the need to declare multiple redundant targets. If make toner is run, it will depend on toner.mml (see below) and will silently copy the output ($< is the expanded name of the first dependency) to project.mml for TileMill to read.

mml: $(subst .yml,.mml,$(filter-out circle.yml,$(wildcard *.yml)))

xml: $(subst .yml,.xml,$(filter-out circle.yml,$(wildcard *.yml)))

Defines targets that will make MML and XML files corresponding to all .yml files except circle.yml (which is a control file for CircleCI, not a style).

.PRECIOUS: %.mml

%.mml: %.yml map.mss labels.mss %.mss interp js-yaml
        @echo Building $@
        @cat $< | interp | js-yaml > tmp.mml && mv tmp.mml $@

Builds *.mml by interpolating environment variables into a Mustache-templated YAML file (interp) and converting to JSON (js-yaml). tmp.mml is used so that mv can atomically move the file into place (without doing this, TileMill periodically chokes when reading partially-written files).

This depends on <style>.yml, map.mss, labels.mss, and <style.mss> so that it will be considered stale when any of those are modified. interp and js-yaml are explicitly called out as dependencies so that they can be installed if necessary (see below).

This is marked as .PRECIOUS so that artifacts won't be deleted when called as an intermediate target (i.e. from an XML target).

.PRECIOUS: %.xml

%.xml: %.mml carto
        @echo
        @echo Building $@
        @echo
        @carto -l $< > $@ || (rm -f $@; false)

Builds *.xml from <style>.mml (declared above). || (rm -f $@; false) is included because carto may leave behind invalid XML files when it fails (and because we want to pass the failure through and terminate the current make invocation.

.PHONY: carto

carto: node_modules/carto/package.json

.PHONY: interp

interp: node_modules/interp/package.json

.PHONY: js-yaml

js-yaml: node_modules/js-yaml/package.json

node_modules/carto/package.json: PKG = $(word 2,$(subst /, ,$@))
node_modules/carto/package.json: node_modules/millstone/package.json
        @type node > /dev/null 2>&1 || (echo "Please install Node.js" && false)
        @echo "Installing $(PKG)"
        @npm install $(PKG)

node_modules/interp/package.json: PKG = $(word 2,$(subst /, ,$@))
node_modules/interp/package.json:
        @type node > /dev/null 2>&1 || (echo "Please install Node.js" && false)
        @echo "Installing $(PKG)"
        @npm install $(PKG)

node_modules/js-yaml/package.json: PKG = $(word 2,$(subst /, ,$@))
node_modules/js-yaml/package.json:
        @type node > /dev/null 2>&1 || (echo "Please install Node.js" && false)
        @echo "Installing $(PKG)"
        @npm install $(PKG)

node_modules/millstone/package.json: PKG = $(word 2,$(subst /, ,$@))
node_modules/millstone/package.json:
        @type node > /dev/null 2>&1 || (echo "Please install Node.js" && false)
        @echo "Installing $(PKG)"
        @npm install $(PKG)

Artificial (.PHONY) targets for required commands along with file-based dependencies and checks for Node. PKG is defined as a make variable in preparation for future refactoring that turns this boilerplate into a macro.

package.json declares dependencies on these commands, but it also includes everything else to run a rendering node, so this is a lower-impact way of ensuring that they're installed.

.PHONY: DATABASE_URL

DATABASE_URL:
        @test "${$@}" || (echo "$@ is undefined" && false)

A target definition that can be used when one wants to ensure that a DATABASE\_URL was provided.

.PHONY: db

db: DATABASE_URL
        @psql -c "SELECT 1" > /dev/null 2>&1 || \
        createdb

Target to ensure that a database exists. If psql returns fall, createdb will be run (using libpq environment variables extracted from DATABASE\_URL) to create one.

.PHONY: db/postgis

db/postgis: db
        $(call create_extension)

.PHONY: db/hstore

db/hstore: db
        $(call create_extension)

Targets that create extensions (and require that a database exists) using the macros defined above.

.PHONY: db/shared

db/shared: db/postgres db/shapefiles

.PHONY: db/postgres

db/postgres: db/functions/highroad

Meta-targets that depend on both explicit (db/shapefiles) and implicit (db/functions/highroad) targets.

.PHONY: db/shapefiles

db/shapefiles: shp/osmdata/land-polygons-complete-3857.zip \
               shp/natural_earth/ne_50m_land-merc.zip \
               shp/natural_earth/ne_50m_admin_0_countries_lakes-merc.zip \
               shp/natural_earth/ne_10m_admin_0_countries_lakes-merc.zip \
               shp/natural_earth/ne_10m_admin_0_boundary_lines_map_units-merc.zip \
               shp/natural_earth/ne_50m_admin_1_states_provinces_lines-merc.zip \
               shp/natural_earth/ne_10m_geography_marine_polys-merc.zip \
               shp/natural_earth/ne_50m_geography_marine_polys-merc.zip \
               shp/natural_earth/ne_110m_geography_marine_polys-merc.zip \
               shp/natural_earth/ne_10m_airports-merc.zip \
               shp/natural_earth/ne_10m_roads-merc.zip \
               shp/natural_earth/ne_10m_lakes-merc.zip \
               shp/natural_earth/ne_50m_lakes-merc.zip \
               shp/natural_earth/ne_10m_admin_0_boundary_lines_land-merc.zip \
               shp/natural_earth/ne_50m_admin_0_boundary_lines_land-merc.zip \
               shp/natural_earth/ne_10m_admin_1_states_provinces_lines-merc.zip

Meta-target for processed Shapefiles.

# TODO places target that lists registered places
PLACES=BC:data/extract/north-america/ca/british-columbia-latest.osm.pbf \
       CA:data/extract/north-america/us/california-latest.osm.pbf \
       belize:data/extract/central-america/belize-latest.osm.pbf \
       cle:data/metro/cleveland_ohio.osm.pbf \
       MA:data/extract/north-america/us/massachusetts-latest.osm.pbf \
       NY:data/extract/north-america/us/new-york-latest.osm.pbf \
       OH:data/extract/north-america/us/ohio-latest.osm.pbf \
       sf:data/metro/san-francisco.osm.pbf \
       sfbay:data/metro/sf-bay-area.osm.pbf \
       seattle:data/metro/seattle_washington.osm.pbf \
       WA:data/extract/north-america/us/washington-latest.osm.pbf

$(foreach place,$(PLACES),$(eval $(call import,$(place))))

Define a list of places along with the local reference to their corresponding extract (data/extract/% and data/metro/% are patterns defined below) and generate import targets (e.g. db/BC for British Columbia).

.SECONDARY: data/extract/%

data/extract/%:
        @mkdir -p $$(dirname $@)
        curl -Lf http://download.geofabrik.de/$(@:data/extract/%=%) -o $@

.SECONDARY: data/metro/%

data/metro/%:
        @mkdir -p $$(dirname $@)
        curl -Lf https://s3.amazonaws.com/metro-extracts.mapzen.com/$(@:data/metro/%=%) -o $@

OSM extract patterns; i.e. anything under data/extract/ will be downloaded from Geofabrik, anything under data/metro/ from Mapzen's Metro Extracts).

These are marked as .SECONDARY to prevent them from being deleted (as they're "expensive" to create). Note: this isn't quite right; my intention is to keep them around but also to delete them if the target failed.

mkdir -p is used to ensure that the target directory exists. $$(dirname $@) is used to pass the literal $(dirname data/metro/<whatever>) to mkdir.

$(@:data/metro/%=%) substitutes <whatever> for data/metro/<whatever> in the target name.

curl's -f option is provided so that it will return with a non-zero exit code on failure and cause the target to fail.

.SECONDARY: data/osmdata/land_polygons.zip

# so the zip matches the shapefile name
data/osmdata/land_polygons.zip:
        @mkdir -p $$(dirname $@)
        curl -Lf http://data.openstreetmapdata.com/land-polygons-complete-3857.zip -o $@

shp/osmdata/%.shp \
shp/osmdata/%.dbf \
shp/osmdata/%.prj \
shp/osmdata/%.shx: data/osmdata/%.zip
        @mkdir -p $$(dirname $@)
        unzip -ju $< -d $$(dirname $@)

shp/osmdata/land_polygons.index: shp/osmdata/land_polygons.shp
        shapeindex $<

.SECONDARY: data/osmdata/land-polygons-complete-3857.zip

shp/osmdata/land-polygons-complete-3857.zip: shp/osmdata/land_polygons.shp \
        shp/osmdata/land_polygons.dbf \
        shp/osmdata/land_polygons.prj \
        shp/osmdata/land_polygons.shx \
        shp/osmdata/land_polygons.index
        zip -j $@ $^

Similar to the OSM extracts, but for a specific file with a non-matching source name. Some of this remains as a relic of when we were using different versions of the land polygons.

define natural_earth
db/$(strip $(word 1, $(subst :, ,$(1)))): $(strip $(word 2, $(subst :, ,$(1)))) db/postgis
        psql -c "\d $$(subst db/,,$$@)" > /dev/null 2>&1 || \
        ogr2ogr --config OGR_ENABLE_PARTIAL_REPROJECTION TRUE \
                        --config SHAPE_ENCODING WINDOWS-1252 \
                        --config PG_USE_COPY YES \
                        -nln $$(subst db/,,$$@) \
                        -t_srs EPSG:3857 \
                        -lco ENCODING=UTF-8 \
                        -nlt PROMOTE_TO_MULTI \
                        -lco POSTGIS_VERSION=2.0 \
                        -lco GEOMETRY_NAME=geom \
                        -lco SRID=3857 \
                        -lco PRECISION=NO \
                        -clipsrc -180 -85.05112878 180 85.05112878 \
                        -segmentize 1 \
                        -skipfailures \
                        -f PGDump /vsistdout/ \
                        /vsizip/$$</$(strip $(word 3, $(subst :, ,$(1)))) | psql -q

shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.shp \
        shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.dbf \
        shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.prj \
        shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.shx: $(strip $(word 2, $(subst :, ,$(1))))
        @mkdir -p $$$$(dirname $$@)
        ogr2ogr --config OGR_ENABLE_PARTIAL_REPROJECTION TRUE \
                        --config SHAPE_ENCODING WINDOWS-1252 \
                        -t_srs EPSG:3857 \
                        -lco ENCODING=UTF-8 \
                        -clipsrc -180 -85.05112878 180 85.05112878 \
                        -segmentize 1 \
                        -skipfailures $$@ /vsizip/$$</$(strip $(word 3, $(subst :, ,$(1))))

shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.index: shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.shp
        shapeindex $$<

.SECONDARY: shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.zip

shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.zip: shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.shp \
        shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.dbf \
        shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.prj \
        shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.shx \
        shp/natural_earth/$(strip $(word 1, $(subst :, ,$(1))))-merc.index
        zip -j $$@ $$^
endef

Macro definition that creates db/ne\_<whatever> and shp/natural\_earth/* targets for Natural Earth sources. It assumes that it's called with arguments in the form <name>:<source file>:[shapefile]. (If someone can help simplify the repeated $(strip $(word 1, $(subst :, ,$(1)))) declarations (used to extract the first component), I'd appreciate it!)

The db/<whatever> target runs ogr2ogr with a set of options that we've found to work well with the Natural Earth Shapefiles over the years.

shp/natural\_earth/*-merc.{shp,dbf,prj,shx} states that there will be 4 artifacts for each invocation of ogr2ogr (used to reproject here). Again, we use options gathered over the years along with /vsizip to avoid needing to unzip. If a single "file" (base name, really) exists in the zip, just the name of the zip file is necessary, otherwise a path to the Shapefile within the zip is necessary (the list of layers below takes advantage of that by allowing [shapefile] to be optional. (This is necessary due to errors in the packaging of some of the Natural Earth layers.)

Aggressive escaping is necessary in order for literal $s to be passed through, e.g. $$$$(dirname $$@).

The shp/natural\_earth/*-merc.zip target uses $^ for the list of files to compress, which contains the expanded names of all of the dependencies. zip's -j option is used to "junk paths" and put everything in the root of the zip file.

# <name>:<source file>:[shapefile]
NATURAL_EARTH=ne_50m_land:data/ne/50m/physical/ne_50m_land.zip \
        ne_50m_admin_0_countries_lakes:data/ne/50m/cultural/ne_50m_admin_0_countries_lakes.zip \
        ne_10m_admin_0_countries_lakes:data/ne/10m/cultural/ne_10m_admin_0_countries_lakes.zip \
        ne_10m_admin_0_boundary_lines_map_units:data/ne/10m/cultural/ne_10m_admin_0_boundary_lines_map_units.zip \
        ne_50m_admin_1_states_provinces_lines:data/ne/50m/cultural/ne_50m_admin_1_states_provinces_lines.zip \
        ne_10m_geography_marine_polys:data/ne-stamen/10m/physical/ne_10m_geography_marine_polys.zip \
        ne_50m_geography_marine_polys:data/ne-stamen/50m/physical/ne_50m_geography_marine_polys.zip \
        ne_110m_geography_marine_polys:data/ne-stamen/110m/physical/ne_110m_geography_marine_polys.zip \
        ne_10m_airports:data/ne-stamen/10m/cultural/ne_10m_airports.zip \
        ne_10m_roads:data/ne/10m/cultural/ne_10m_roads.zip \
        ne_10m_lakes:data/ne/10m/physical/ne_10m_lakes.zip \
        ne_50m_lakes:data/ne/50m/physical/ne_50m_lakes.zip \
        ne_10m_admin_0_boundary_lines_land:data/ne/10m/cultural/ne_10m_admin_0_boundary_lines_land.zip \
        ne_50m_admin_0_boundary_lines_land:data/ne/50m/cultural/ne_50m_admin_0_boundary_lines_land.zip \
        ne_10m_admin_1_states_provinces_lines:data/ne/10m/cultural/ne_10m_admin_1_states_provinces_lines.zip:ne_10m_admin_1_states_provinces_lines.shp

$(foreach shape,$(NATURAL_EARTH),$(eval $(call natural_earth,$(shape))))

Define a list of Natural Earth layers along with their local file references and (optional) Shapefile names and call natural_earth to generate targets. As with the OSM extracts, patterns are used to match the conventions used by the Natural Earth site.

define natural_earth_sources
.SECONDARY: data/ne/$(1)/$(2)/%.zip

data/ne/$(1)/$(2)/%.zip:
        @mkdir -p $$(dir $$@)
        curl -fL http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/$(1)/$(2)/$$(@:data/ne/$(1)/$(2)/%=%) -o $$@

.SECONDARY: data/ne/$(1)/$(2)/%.zip

data/ne-stamen/$(1)/$(2)/%.zip:
        @mkdir -p $$(dir $$@)
        curl -fL "https://github.com/stamen/natural-earth-vector/blob/master/zips/$(1)_$(2)/$$(@:data/ne-stamen/$(1)/$(2)/%=%)?raw=true" -o $$@
endef

Macro definition for meta Natural Earth patterns.

scales=10m 50m 110m
themes=cultural physical raster

$(foreach a,$(scales),$(foreach b,$(themes),$(eval $(call natural_earth_sources,$(a),$(b)))))

Generate targets for nested combinations of scales and themes.

# complete wrapping
else
.DEFAULT:
        $(error Please install pgexplode: "npm install pgexplode")
endif

Provide a default target (truly default in that it will execute for any requested target) explaining what needs to be done for things to work correctly.

Questions? Suggestions?

Seth Fitzsimmons

github.com/mojodna

seth@mojodna.net