diff --git a/src/Makefile.am b/src/Makefile.am
index 4e43da2..7901177 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -10,6 +10,7 @@ libapfl_a_SOURCES += context.c
 libapfl_a_SOURCES += error.c
 libapfl_a_SOURCES += eval.c
 libapfl_a_SOURCES += expr.c
+libapfl_a_SOURCES += format.c
 libapfl_a_SOURCES += gc.c
 libapfl_a_SOURCES += hashmap.c
 libapfl_a_SOURCES += internal.c
@@ -28,6 +29,7 @@ apfl_internal_headers += alloc.h
 apfl_internal_headers += bytecode.h
 apfl_internal_headers += compile.h
 apfl_internal_headers += context.h
+apfl_internal_headers += format.h
 apfl_internal_headers += gc.h
 apfl_internal_headers += hashmap.h
 apfl_internal_headers += internal.h
diff --git a/src/apfl.h b/src/apfl.h
index 1366bc2..d5b0691 100644
--- a/src/apfl.h
+++ b/src/apfl.h
@@ -191,7 +191,7 @@ struct apfl_error {
     char byte;
 };
 
-void apfl_error_print(struct apfl_error, FILE *);
+bool apfl_error_print(struct apfl_error, FILE *);
 
 struct apfl_error apfl_error_simple(enum apfl_error_type);
 bool apfl_error_is_fatal_type(enum apfl_error_type);
diff --git a/src/error.c b/src/error.c
index 8621aa4..52b1165 100644
--- a/src/error.c
+++ b/src/error.c
@@ -1,11 +1,15 @@
- #include <stdio.h>
+#include <stdio.h>
 
 #include "apfl.h"
 
+#include "format.h"
+
 #define POSFMT "%d:%d"
 #define POSARGS error.position.line, error.position.col
 #define POS2ARGS error.position2.line, error.position2.col
 
+#define TRY(x) do { if (!(x)) return false; } while (0)
+
 const char *
 apfl_error_type_name(enum apfl_error_type type)
 {
@@ -61,132 +65,136 @@ apfl_error_type_name(enum apfl_error_type type)
     return "<unknown error>";
 }
 
-void
-apfl_error_print(struct apfl_error error, FILE *file)
+static bool
+format_pos(struct apfl_format_writer w, struct apfl_position pos)
+{
+    TRY(apfl_format_put_int(w, pos.line));
+    TRY(apfl_format_put_string(w, ":"));
+    TRY(apfl_format_put_int(w, pos.col));
+    return true;
+}
+
+static bool
+format_error(struct apfl_format_writer w, struct apfl_error error)
 {
     switch (error.type) {
     case APFL_ERR_MALLOC_FAILED:
-        fprintf(file, "Could not allocate memory\n");
-        return;
+        return apfl_format_put_string(w, "Could not allocate memory");
     case APFL_ERR_INPUT_ERROR:
-        fprintf(file, "Input error while parsing\n");
-        return;
+        return apfl_format_put_string(w, "Input error while parsing");
     case APFL_ERR_UNEXPECTED_EOF:
-        fprintf(file, "Unexpected end of file\n");
-        return;
+        return apfl_format_put_string(w, "Unexpected end of file");
     case APFL_ERR_EXPECTED_EQ_AFTER_COLON:
-        fprintf(file, "Expected '=' after ':' at " POSFMT "\n", POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "Expected '=' after ':' at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_UNEXPECTED_BYTE:
-        fprintf(file, "Unexpected byte '%c' (0x%X) at " POSFMT "\n", error.byte, (unsigned)error.byte, POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "Unexpected byte '"));
+        TRY(apfl_format_put_char(w, error.byte));
+        TRY(apfl_format_put_string(w, "' (0x"));
+        TRY(apfl_format_put_hexbyte(w, (unsigned char)error.byte));
+        TRY(apfl_format_put_string(w, ") at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_UNEXPECTED_BYTE_IN_NUMBER:
-        fprintf(file, "Unexpected byte '%c' while parsing number at " POSFMT "\n", error.byte, POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "Unexpected byte '"));
+        TRY(apfl_format_put_char(w, error.byte));
+        TRY(apfl_format_put_string(w, "' while parsing number at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_EXPECTED_DIGIT:
-        fprintf(file, "Expected a digit at " POSFMT "\n", POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "Expected a digit at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_EXPECTED_HEX_IN_HEX_ESCAPE:
-        fprintf(file, "Expected a hex-digit in hex escape at " POSFMT "\n", POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "Expected a hex-digit in hex escape at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_INVALID_ESCAPE_SEQUENCE:
-        fprintf(file, "Invalid escape sequence \\%c at " POSFMT "\n", error.byte, POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "Invalid escape sequence \\"));
+        TRY(apfl_format_put_char(w, error.byte));
+        TRY(apfl_format_put_string(w, " at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_NO_LINEBREAK_AFTER_CONTINUE_LINE:
-        fprintf(file, "No line break (after optional comments) after \\ at " POSFMT "\n", POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "No line break (after optional comments) after \\ at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_UNEXPECTED_TOKEN:
-        fprintf(file, "Unexpected `%s` token at " POSFMT "\n", apfl_token_type_name(error.token_type), POSARGS);
-        return;
+        TRY(apfl_format_put_string(w, "Unexpected `"));
+        TRY(apfl_format_put_string(w, apfl_token_type_name(error.token_type)));
+        TRY(apfl_format_put_string(w, "` token at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_MISMATCHING_CLOSING_BRACKET:
-        fprintf(
-            file,
-            "Closing `%s` token at " POSFMT " does not match opening `%s` at " POSFMT "\n",
-            apfl_token_type_name(error.token_type),
-            POSARGS,
-            apfl_token_type_name(error.token_type2),
-            POS2ARGS
-        );
-        return;
+        TRY(apfl_format_put_string(w, "Closing `"));
+        TRY(apfl_format_put_string(w, apfl_token_type_name(error.token_type)));
+        TRY(apfl_format_put_string(w, "` token at "));
+        TRY(format_pos(w, error.position));
+        TRY(apfl_format_put_string(w, "Does not match opening `"));
+        TRY(apfl_format_put_string(w, apfl_token_type_name(error.token_type2)));
+        TRY(apfl_format_put_string(w, "` at "));
+        TRY(format_pos(w, error.position2));
+        return true;
     case APFL_ERR_UNEXPECTED_EOF_AFTER_TOKEN:
-        fprintf(
-            file,
-            "Unexpected end of file after `%s` token at " POSFMT "\n",
-            apfl_token_type_name(error.token_type),
-            POSARGS
-        );
-        return;
+        TRY(apfl_format_put_string(w, "Unexpected end of file after `"));
+        TRY(apfl_format_put_string(w, apfl_token_type_name(error.token_type)));
+        TRY(apfl_format_put_string(w, "` token at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_STATEMENTS_BEFORE_PARAMETERS:
-        fprintf(
-            file,
-            "Unexpected statements before parameters near " POSFMT "\n",
-            POSARGS
-        );
-        return;
+        TRY(apfl_format_put_string(w, "Unexpected statements before parameters near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_EMPTY_ASSIGNMENT_BEFORE_PARAMETERS:
-        fprintf(
-            file,
-            "Unexpected empty assignment before parameters near " POSFMT "\n",
-            POSARGS
-        );
-        return;
+        TRY(apfl_format_put_string(w, "Unexpected empty assignment before parameters near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_UNEXPECTED_EXPRESSION:
-        fprintf(
-            file,
-            "Unexpected expression near " POSFMT "\n",
-            POSARGS
-        );
-        return;
+        TRY(apfl_format_put_string(w, "Unexpected expression near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_INVALID_ASSIGNMENT_LHS:
-        fprintf(
-            file,
-            "Invalid left hand side of assignment near " POSFMT "\n",
-            POSARGS
-        );
-        return;
+        TRY(apfl_format_put_string(w, "Invalid left hand side of assignment near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_EMPTY_ASSIGNMENT:
-        fprintf(
-            file,
-            "Empty assignment at " POSFMT "\n",
-            POSARGS
-        );
-        return;
+        TRY(apfl_format_put_string(w, "Empty assignment at "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_ONLY_ONE_EXPAND_ALLOWED:
-        fprintf(
-            file,
-            "Only one expansion (~) is allowed per level, near " POSFMT "\n",
-            POSARGS
-        );
-        break;
+        TRY(apfl_format_put_string(w, "Only one expansion (~) is allowed per level, near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_UNEXPECTED_CONSTANT_IN_MEMBER_ACCESS:
-        fprintf(
-            file,
-            "Unexpected constant in member access near " POSFMT "\n",
-            POSARGS
-        );
-        break;
+        TRY(apfl_format_put_string(w, "Unexpected constant in member access near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_UNEXPECTED_EXPR_IN_MEMBER_ACCESS:
-        fprintf(
-            file,
-            "Unexpected expression in member access near " POSFMT "\n",
-            POSARGS
-        );
-        break;
+        TRY(apfl_format_put_string(w, "Unexpected expression in member access near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_UNEXPECTED_BLANK_IN_MEMBER_ACCESS:
-        fprintf(
-            file,
-            "Unexpected blank (\"_\") in member access near " POSFMT "\n",
-            POSARGS
-        );
-        break;
+        TRY(apfl_format_put_string(w, "Unexpected blank (\"_\") in member access near "));
+        TRY(format_pos(w, error.position));
+        return true;
     case APFL_ERR_NOT_IMPLEMENTED:
-        fprintf(
-            file,
-            "Feature not implemented\n"
-        );
+        TRY(apfl_format_put_string(w, "Feature not implemented"));
+        return true;
     }
 
-    fprintf(file, "Unknown error %d\n", (int)error.type);
+    TRY(apfl_format_put_string(w, "Unknown error "));
+    TRY(apfl_format_put_int(w, (int)error.type));
+    return true;
+}
+
+bool
+apfl_error_print(struct apfl_error error, FILE *file)
+{
+    struct apfl_format_writer w = apfl_format_file_writer(file);
+    TRY(format_error(w, error));
+    TRY(apfl_format_put_char(w, '\n'));
+    return true;
 }
 
 struct apfl_error
diff --git a/src/format.c b/src/format.c
new file mode 100644
index 0000000..ecf1c7a
--- /dev/null
+++ b/src/format.c
@@ -0,0 +1,61 @@
+#include <assert.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+
+#include "apfl.h"
+
+#include "format.h"
+
+#define WRITE(w, buf, len) w.write(w.opaque, buf, len)
+#define TRY(x) if (!(x)) return false;
+
+static bool
+write_file(void *opaque, const char *buf, size_t len)
+{
+    FILE *f = opaque;
+    return fwrite(buf, len, 1, f) == 1;
+}
+
+struct apfl_format_writer
+apfl_format_file_writer(FILE *f)
+{
+    return (struct apfl_format_writer) {
+        .write = write_file,
+        .opaque = f,
+    };
+}
+
+bool
+apfl_format_put_string_view(struct apfl_format_writer w, struct apfl_string_view sv)
+{
+    return WRITE(w, sv.bytes, sv.len);
+}
+
+#define PUT_INT_BUFSIZE 21 // ceil(log10(2**64)) + 1
+
+bool
+apfl_format_put_int(struct apfl_format_writer w, int i)
+{
+    char buf[PUT_INT_BUFSIZE];
+    size_t len = snprintf(buf, PUT_INT_BUFSIZE, "%d", i);
+    assert(len < PUT_INT_BUFSIZE);
+    return WRITE(w, buf, len);
+}
+
+bool
+apfl_format_put_char(struct apfl_format_writer w, char c)
+{
+    return WRITE(w, &c, 1);
+}
+
+#define PUT_HEXBYTE_BUFSIZE 3
+
+bool
+apfl_format_put_hexbyte(struct apfl_format_writer w, unsigned char c)
+{
+    char buf[PUT_HEXBYTE_BUFSIZE];
+    size_t len = snprintf(buf, PUT_HEXBYTE_BUFSIZE, "%x", (unsigned) c);
+    assert(len < PUT_HEXBYTE_BUFSIZE);
+    return WRITE(w, buf, len);
+}
diff --git a/src/format.h b/src/format.h
new file mode 100644
index 0000000..2157def
--- /dev/null
+++ b/src/format.h
@@ -0,0 +1,31 @@
+#ifndef APFL_FORMAT_H
+#define APFL_FORMAT_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdio.h>
+
+#include "apfl.h"
+
+struct apfl_format_writer {
+    bool (*write)(void *, const char *buf, size_t len);
+    void *opaque;
+};
+
+struct apfl_format_writer apfl_format_file_writer(FILE *f);
+
+bool apfl_format_put_string_view(struct apfl_format_writer, struct apfl_string_view);
+#define apfl_format_put_string(w, s) apfl_format_put_string_view((w), apfl_string_view_from(s))
+bool apfl_format_put_int(struct apfl_format_writer, int);
+bool apfl_format_put_char(struct apfl_format_writer, char);
+bool apfl_format_put_hexbyte(struct apfl_format_writer, unsigned char);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif